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AUTOMOTIVE MAINTENANCE TECHNOLOGY SUBJECT: - BRAKING SYSTEM 1

TITLE: - HYDRAULIC BRAKE

Semester: - 2

Duration: - 6 Page: - 33/ 33

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PRINCIPLES OF HYDRAULIC BRAKE SYSTEMS

By late 1920's / early 1930's most cars were equipped with hydraulic actuated brakes.

This greatly increased braking reliability and safety.

A hydraulic system (Figure 1) uses a brake fluid to transfer pressure from the brake pedal to the pads or shoes.

This transfer of pressure is reliable and consistent because liquids are not compressible.

That is, pressure applied to a liquid in a closed system is transmitted by that liquid equally to every other part of that system.

For example, move piston 1 a distance of 1 inch and piston 2 also moves 1 inch (Figure 2). The force can be increased at output (that is, at the wheel) by increasing the size of the wheel's piston, though piston travel decreases.

The force at output can be decreased by decreasing the size of the wheel piston, but the piston travel increases.

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Thus, to double the output force of the 5 psi at the master cylinder to 10 psi at the wheels, simply use a wheel cylinder piston with a surface area of 2 square inches (Figure 3).

To triple the force of 100 psi, use a piston with 3 square inches and 300 pounds of output result (Figure 4).

No matter what the fluid pressure is, the output force can be increased with a larger piston, though piston travel eases proportionately.




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In actual practice, however fluid movement in an automotive hydraulic brake system is very slight.

In an emergency, when the pedal goes all the way to the floor, the volume of fluid displaced amounts to only about 20 cubic centimeters. About 15 cubic centimeters goes to the front discs and 5 cubic centimeters goes to the rear drum.

Of course, the hydraulic system does not stop the car all by itself. In fact, it really just transmits the action of the driver's foot on the brake pedal out to the wheels figure 5.




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In the wheels, sets of friction pads are against rotors or drums to slow their turning and bring the car to a stop.

Mechanical force (the driver stepping on the brake pedal) is changed into hydraulic pressure which is changed back into mechanical force (brake shoes and disc pads contacting the drums and rotor)

The amount of force acting on the friction pads shoes is equivalent to the psi applied to the pedal multiplied by the area of the piston affected.

A force of 25 psi applied to the pedal times 4 square inches of pis- area equals 100 pounds of pressure in the system figure 6

Single Braking System

With very few exceptions, the single circuit hydraulic system (Figure 7) was used on cars made through 1966.

This system used a master cylinder with one piston that operated the brakes on all four wheels.

Later, its use was confined to trucks and industrial and construction equipment.




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Dual Braking Systems

Since 1967 all cars be equipped with two separate brake systems.

If one circuit fails, the other provides enough braking power to safely stop the car.

The dual system differs from the single system by employing a tandem master cylinder, which is essentially two master cylinders formed by installing two separate pistons and fluid reservoirs into one cylinder bore.

Each piston applies hydraulic pressure to two wheels.

Front / Rear Split System

In early dual systems, the hydraulic circuits were separated front and rear (Figure 8).

Both front wheels were on one hydraulic circuit and both rear wheels on another.

If a failure occurred in one system, the other system was still available to stop the vehicle.




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However, the front brakes do approximately 70 percent of the braking work.

A failure in the front brake system would only leave 20 to 40 percent braking power.

This problem was somewhat reduced, with the development of diagonally split systems.

Diagonally Split System

The diagonally split system operates on the same principles as the front and rear split system.

It uses primary and secondary master cylinders that move simultaneously to exert hydraulic pressure on their respective systems.

The hydraulic brake lines on this system, however, have been diagonally split front to rear (left front to right rear and right front to left rear) (Figure 9) .

The circuit split can occur within the master cylinder or externally at a proportioning valve or pressure differential switch.




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In the event of a system failure, the remaining good system would do all the braking on one front wheel and the opposite rear wheel, thus maintaining 50 percent of the total braking force.

HYDRAULIC BRAKE SYSTEM COMPONENTS

Brake Fluid

Brake fluid is the lifeblood of any hydraulic brake system and is what makes the system operate properly.

Modern brake fluid is specially blended to enable it to perform a variety of functions.

Brake fluid must be able to flow freely at extremely high temperatures (500°F) and at very low temperatures (-104°F).

Brake fluid must also serve as a lubricant to the many parts with which it comes into contact to ensure smooth and even operation/ In addition, brake fluid must fight corrosion and rust in the brake lines and various assemblies and components in services.




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An- other important property of brake fluid is that it must resist evaporation. All brake fluids are hydroscopic; that is, they readily absorb water.

This is why brake fluid should always be kept in a sealed container and should only be exposed to outside air for limited periods of time.

Some of the earliest brake fluids had chemicals in them that ate away at the rubber components in the brake system (i.e., cups and seals).

Modern brake fluid must be compatible with rubber to avoid damage to the cups and seals in the system.

Brake fluid must provide a controlled amount of swell to the brake system cups and seals.

There must be just enough swell to form a good seal. However, the swell cannot be too great. If it is, drag and poor brake response occur.

Every can of brake fluid cames the identification letters of SAE and DOT.

These letters (and corresponding numbers) indicate the nature, blend, and performance characteristics of that particular brand of brake fluid.

There are three basic types or classifications of hydraulic brake fluids. DOT 3 is a conventional brake fluid with a minimum dry ERBP (equilibrium reflux boiling point) of 401°F and a minimum wet ERBP of 284°F.

It is generally recommended for most ABS systems and some power brake setups.

DOT 4 is a conventional brake fluid with & minimum dry ERBP of 446°F and a minimum wet ERBP of 356°F.

It is the most commonly used brake fluid for conventional brake systems.

DOT 5 is a unique silicone-based brake fluid with a minimum dry ERBP of 500°F and a minimum WET ERBP of 356°F.

In the last few years, DOT 5 has lost its demand by brake servicing experts.

However, remember that it is best to follow the vehicle manufacturer's recommendations.

Some vehicles have brake fluid level sensors that provide the driver with an early warning message at the brake fluid in the master cylinder reservoir has dropped below the normal level.

As the brake fluid in the master cylinder reservoir drops below the designated level, the sensor closes the warning message circuit.

About 15 S seconds later, the message "brake fluid low" appears on the instrument panel.




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At this time, the master cylinder reservoir should be checked and filled to the correct level with the specified brake fluid.

WARNING! Use only approved brake fluid in a brake stem. Any other lubricant, such as power steering fluid, automatic transmission fluid, or engine oil, which has a petroleum base, must never be used in the brake system. Petroleum-based fluids attack the rubber components in the brake system, like the piston cups and seals, and cause them to swell and disintegrate.

Brake Pedal

Brake pedal is where the brake's hydraulic system gets its start. When the brake pedal is depressed, force is applied to the master

cylinder. On a basic hydraulic brake system (where there is no power assist),

the force applied is transmitted mechanically. As the pedal in Figure 10 pivots, the force applied it is multiplied

mechanically. The force that the pushrod applies to the master cylinder piston is,

therefore, much greater than the force applied to the brake pedal.




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Master Cylinders

The heart of the hydraulic brake system is the master Cylinder. It converts the mechanical force of the driver's foot on the brake pedal

to hydraulic pressure. The master cylinder has ,a bore that contains a piston assembly and

return spring. Seals on the piston prevent fluid leakage. A cup seal on the forward end keeps the brake fluid ahead of the

piston when it is put under pressure. Although master cylinders differ in number of pistons, reservoir design,

and integrated hydraulic components, the operation of all master cylinders is basically the same.

Master cylinders are generally constructed of cast iron (Figure 11A) with an integral fluid reservoir or weight -saving aluminum with a separate molded nylon or fiberglass reinforced plastic reservoir.

Aluminum body master cylinders feature an anodized body (Figure 11B) to protect against corrosion and to extend bore life

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WARNING! It is recommended that aluminum master cylinders not be rebuilt if pitting or scoring of the cylinder bore is evident. A new unit should be installed.

1. Single System Master Cylinders To understand how a master cylinder works, it is easier to

examine a single piston master cylinder before studying dual master cylinders.

A cross section of a typical single piston cylinder is shown in Figure 12.

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The cylinder and the fluid reservoir are an integral unit. A vent port and compensating or replenishing port pass from the

reservoir into the piston chamber. The chamber contains a piston, a return spring, and a residual

pressure check valve. On the brake pedal end of the cylinder, a pushrod connects the

cylinder to the power booster or brake pedal clevis.

The opposite end of the cylinder contains the threaded fluid outlet to which the hydraulic line is connected.

When the brake pedal is depressed, the force from the brake pedal is transferred to the piston by a pushrod.

The piston exerts pressure on the fluid, which flows out through the fluid outlet port into a single hydraulic system, serving both front and rear brakes.

When the brake pedal is released, the return spring in the cylinder forces the piston back to its original position.

If the piston returns too quickly, vacuum is generated in the cylinder.

Fluid from the reservoir then enters the cylinder through a vent port to fill the void.

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As the brakes are fully released, excess fluid is returned to the reservoir through the replenishing or compensating port to relieve pressure in the system.

Single piston master cylinders have not been used in passenger vehicles for many years. Safety considerations dictated the use of dual ( one cylinder for two wheels) master cylinders.

SHOP TALK Master cylinder terminology is often confusing. For example, replenishing and compensating ports are synonymous terms. That is, they serve the same function but are called different names by different manufacturers and in different parts of the country. The same is true of the master cylinder's vent port, by-pass port, or relief passage. All these things actually perfom the same function and in basically the same manner, but are called by different names. .

2. Dual Master Cylinders

The important parts of a typical dual master cylinder are shown in Figure 13.

Their functions are described here. The cap is also known as the bail, cover, or gasket.

This part keeps an airtight seal on the master cylinder while letting it breathe.

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It keeps moisture from entering the system. Moisture breaks down the ability of brake fluid to lubricate

properly. It also causes corrosion. The cap, or cover gasket, acts like a

diaphragm. It expands and contracts as the fluid pressure rises or falls. When the brakes are applied, the demand for brake fluid could

create a vacuum below the diaphragm. This would keep fluid from flowing out of the reservoir. For the gasket to expand or contract, the space between the

gasket and the cover must be vented. The venting is accomplished with small grooves or drilled holes

in the cap or cover. (Most seals have a bellow design. As the fluid goes down, the space above the gasket fills with air. When the brakes are released, the fluid returns to the master cylinder reservoirs and the air is forced out the vent holes. )

The wire bail retainer (Figure 14) or a bolt is typically used to hold on the cap or cover.

Extra brake fluid is stored in two separate reservoirs.

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Made of either cast iron or specially blended plastic, they are designed to function independently as a protection against lost brake fluid which can cause the brake system to fail.

It is important to keep the reservoir filled to its proper level. The dual master cylinder reservoir has a single piston bore that

contains the two pistons (primary and secondary) lined up in tandem.

Each piston has a return spring in front of it. This holds the piston cup slightly behind the replenishing port for each reservoir.

In addition to keeping the replenishing ports uncovered, the return springs also return the brake pedal when the force has been removed from it.

As the brakes are applied, the stiffer primary piston spring pushes the secondary piston and spring slight.

The cup at the front end of the secondary piston ses and closes off the primary replenishing port the secondary side of the master cylinder.

A retaining ring fits into a groove near the end of bore and holds the piston.

The primary outlet (or port) is the one closer to cowl panel. It is off the primary piston (the one which receives the initial, or primary movement).

The function of the residual check valve, found only on vehicles with rear drum brakes, is to keep light (or residual) pressure in the brake lines and also at the wheel cylinders.

Without residual pressure, air can be sucked past the wheel cylinder cups and into the wheel cylinders.

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This could occur when the brake pedal is released quickly. Residual check valves are designed to permit hydraulic fluid

and pressure to flow from the master cylinder into the hydraulic lines when the pressure in the master cylinder becomes great enough to open the valve.

After allowing the fluid to flow back into the master cylinder when the pedal is released, the outlet check valve spring (Figure 15) takes over and keeps the hydraulic lines and wheel cylinders full of fluid, ready for the next application of the brakes.

On most of the newer car models, automobile manufacturers have eliminated residual pressure check valves from the brake system.

Wheel cylinders are equipped with cup expanders. The cup expanders keep the cup seals tightly pressed against

the cylinder walls to prevent air from entering the system. Light residual pressure is needed for operating the drum brakes. However, when the brakes are used repeatedly, too much

pressure can build up. The replenishing or compensating port (Figure 16) takes care of

this problem. After many stops, like in city driving, the brake drums and wheel

cylinders get very hot.

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This causes the brake fluid to heat up and expand. Such a pressure buildup can become so strong that it keeps the

brake shoes from returning after releasing the brake pedal. If the brake shoes do not return properly, they might drag

against the drums. This causes more heat to build up and pressure is further

increased.

SHOP TALK ~ Extra brake fluid is forced into the hydraulic system by pumping the brakes. Pressure builds until the brake pedal is released and the piston moves back far enough to uncover the replenishing ports. This removes the excess pressure from the brake lines and wheel cylinders and allows the fluid to return to the brake master cylinder reservoir.

Vent ports help to pump up a low brake pedal by allowing more fluid in the lines.

There are several reasons for a brake pedal to be lower than normal.

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One is that the clearance between the brake lining and the drum is too large.

This can happen when the automatic brake adjusters, if the vehicle is so equipped, are not working properly.

Another reason could be the result of air leaking into the brake system.

If a brake pedal is pumped too rapidly, the return springs in the master cylinder might move the pistons back very fast.

At the other end of the system, the brake shoe return springs move the wheel cylinder pistons much more slowly.

The flow of brake fluid from the wheel cylinders is then delayed. It cannot match the fast return of the master cylinder pistons. As a result, pressure drops in the master cylinder. The pressure

in the master cylinder actually becomes lower than the pressure in the reservoir.

Fluid then rushes through the vent ports, through holes in the piston, past the piston cup, and back into the area ahead of the piston cups.

Vent ports are important because the replenishing ports are not large enough to handle the extra flow of fluid needed when the pedal is pumped.

The design of the master cylinder used on a particular model car depends on the capacity of the reservoir and the displacement of the pistons at the wheel.

Disc brakes require larger reservoirs than drum brakes because the caliper cylinder is much larger than that of a wheel cylinder.

Some internal parts of the master cylinder may also change with application.

For example, in the 1980s General Motors introduced caliper pistons that had a low-drag.

These were designed to retract farther into the caliper's cylinder when the brake pedal was released.

This reduction of drag increased fuel economy. However, since the piston had to move farther to push the pads

against the rotor, more fluid had to be moved by the master cylinder.

This caused a delay in pedal response and the feeling of a low pedal.

To correct this problem, General Motors uses a quick take up master cylinder, with a larger rear piston to move more fluid.

A quick-take-up valve in the master cylinder controls the flow of fluid from the reservoir to the master cylinder's rear piston.

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OPERATION OF THE DUAL SYSTEM MASTER CYLINDER

Actually, the operation of the dual master cylinder is like that of two single piston types placed end to end in the same bore.

The two pistons in the master cylinder are not rigidly connected.

Each half really operates like a single system. As just mentioned, each piston has its own reservoir with vent and replenishing ports.

Stepping the brake pedal moves a pushrod forward, causing the first or primary piston to move forward.

The fluid ahead of it cannot be compressed, so the secondary piston moves.

As the pistons progress deeper into the cylinder bore, the brake fluid that is put under pressure transmits this force through both systems to friction pads in the wheels (Figure 17).

The pistons have seals at both ends: a cup seal with a one-way sealing property at the forward end and a secondary cup at the rear.

Each piston has a return spring with the primary piston spring between two pistons.

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On the forward stroke, the lip of each cup seal is forced against the bore wall.

When piston springs quickly back to place, the brake fluid cannot return through the lines fast enough to avoid creating a low-pressure condition ahead of it.

Fluid must reach the low-pressure area in time for other stroke of the cylinder.

On the return stroke, the edges of the cup pull away from the bore enough allow fluid to pass around the piston assembly to area of low pressure (Figure 18).

At the same time, the reservoir returns the small amount of fluid it had taken in during piston action (Figure 19).

This way, the master cylinder returns to normal before all the fluid returns from the lines.

Finally, the pads and shoes return to position.

That is, once the brake pedal is released and the master cylinder piston has returned to the rest position, shoe return springs (in the drum brakes) and piston seals (in the disc brakes) cause these pistons to retract.

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HYDRAULIC TUBES AND HOSES

Steel tubing and flexible synthetic rubber hosing serve as the arteries and veins of the hydraulic brake system.

These brake lines transmit brake fluid pressure (the blood) from the master cylinder (the heart) to the wheel cylinders and calipers (the muscles and working parts) of the drum and disc brakes.

Fluid transfer from the driver-actuated master cylinder is usually routed through one or more valves and then into the steel tubing and hoses (Fig. 20).

The design of the brake lines offers quick fluid transfer response with very little friction loss.

Engineering and installing the brake lines so they do not wrap around sharp curves is very important in maintaining this good fluid transfer.

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BRAKE LINE TUBING

Most brake line tubing consists of copper-fused double-wall steel tubing in diameters ranging from 1/8 to 3/8 inch.

Some OEM brake tubing is manufactured with soft steel strips, sheathed with copper.

The strips are rolled into a double-wall assembly and then bonded in a furnace at extremely high temperatures. Corrosion protection is often added by tin-plating the tubing.

FITTINGS

Assorted fittings are used to connect steel tubing to junction blocks or other tubing sections.

The most common fitting is the double or inverted flare style.

Double flaring is important to maintain the strength and safety of the system.

Single flare or sleeve compression fittings may not hold up in the rigorous operating environment of a standard vehicle brake system.

Fittings are constructed of steel or brass. The 37 degree inverted flare or standard flare fitting is the most commonly used coupling.

Newer vehicles may use the ISO or metric bubble flare fitting.

Never change the style of fitting being used on the vehicle.

Replace ISO fittings only with ISO fittings. Replace standard fittings with standard fittings.

The metal composition of the fittings must also match exactly.

Using an aluminum-alloy fitting with steel tubing may provide a good initial seal, but the dissimilar metals create a corrosion cell that eats away the metal and reduces the connection's service life.

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BRAKE LINE HOSES

Brake line hoses offer flexible connections to wheel units so steering and sus- pension members can operate without damaging the brake system.

Typical brake hoses range from 10 to 30 inches in length and are constructed of multiple layers of fabric impregnated with a synthetic rubber.

Brake hose material must offer high heat resistance and withstand harsh operating conditions.

SHOP TALK ~ Many brake hose failures can be traced to errors made in the original installation or repair of the hose. Hoses twisted into place become stressed and are prime candidates for leaks and bursting. Most manufacturers now print a natural lay indicator or line on the hose. By making sure this line is not spiralled after fittings are tightened, you can ensure the hose is not overly stressed. Also, always use a hose of the same length and diameter as the original during servicing to maintain brake balance at all wheels. .

HYDRAULIC SYSTEM SAFETY SWITCHES AND VALVES Switches and valves are installed in the brake system hydraulic lines to act as

warning devices to pressure control devices.

Pressure Differential (Warning Light) Switches

A pressure differential valve is used in all dual brake systems to operate a warning light switch.

Its main purpose is to tell the driver if pressure is lost in either of the two hydraulic systems.

Since each brake hydraulic system functions independently; it is possible the driver might not notice immediately that pressure and braking are lost.

When a pressure loss occurs, brake pedal travel increases and a more-than-usual effort is need for braking.

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Should the driver not notice the extra effort needed, the warning light is actuated by the hydraulic system safety switch.

Under normal conditions, the hydraulic pressure on each side of the pressure differential valve piston is balanced.

The piston is located at its center point, so the spring-loaded warning switch plunger fits into the tapered groove of the piston.

This leaves the contacts of the warning switch open. The brake warning light stays off.

If there is a leak in the front or rear braking system, the hydraulic pressure in the two systems is unequal.

For example, if there is a leak in the system supplying the front brakes, there is lower pressure in the front system when the brake pedal is applied.

The hydraulic pressure in the rear system then pushes the piston toward the front side, where the pressure is lower.

As the piston moves, the plunger is pushed out (figure 21).

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This closes the switch and illuminates the brake warning light.

While all brake warning light switches serve the same function, there are three common variations in the design of these switches.

These variations include :

switches with centering springs, without centering springs, with centering springs and two pistons.

Metering and Proportioning Valves

Metering and proportioning valves are used to balance the braking characteristics of disc and drum brakes.

The braking response of the disc brakes is immediate when the brake pedal is applied.

It is directly proportionate to the effort applied at the pedal.

Drum brake response is delayed while rear brake hydraulic pressure moves the wheel cylinder pistons to over- come the force of their return springs and force the brake shoes to contact the drum.

Their actions are seIf-energizing and tend to multiply the pedal effort.

Metering Valve

A metering valve in the front brake line holds off pressure going from the master cylinder to the front disc calipers.

This delay allows pressure to build up in the rear drums first.

When the rear brakes begin to take hold, the hydraulic pressure builds to the level needed to open the metering valve (Figure 22).

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When the metering valve opens, line pressure is high enough to operate the front discs.

This process provides for better balance of the front and rear brakes.

It also prevents lockup of the front brakes by keeping pressure from them until the rear brakes have started to operate.

The metering valve has the most effect at the start of each brake operation and all during light braking conditions.

Proportioning Valve

The self-energizing action of the delayed response rear drum brakes can cause them to lock the rear wheels at a lower hydraulic pressure than the front brakes.

The proportioning valve is used to control rear brake pressures, particularly during hard stops.

When the pressure to the rear brakes reaches a specified level, the proportioning valve over- comes the force of its spring-loaded piston, stopping the flow of fluid to the rear brakes (Figure 23).

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By doing so, it regulates rear brake system pressure and adjusts for the difference in pressure between front and rear brake systems.

This keeps front and rear braking forces in balance.

SHOP TALK ~ Distribution and weight on some vehicles, such as station wagons, is such that a rear-to-front weight transfer does not present a similar problem. For this reason, standard proportioning valves are not required on all station wagon models. .

Combination Valves

Most newer cars have a combination valve in their hydraulic system.

This valve is simply a single unit that combines the metering and proportioning valves with the pressure differential valve.

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Combination valves are described as three-function or two-function valves, depending on the number of functions they perform in the hydraulic system.

THREE-FUNCTION VALVE

This type of valve (Figure 24) performs the functions of the metering valve, brake warning light switch, and proportioning valve.

TWO-FUNCTION VALVES

There are two variations of the two-function combination valve.

One variation does the proportioning valve and brake warning light switch functions.

The other performs the metering valve and brake warning light switch functions.

If anyone of its several operation fails, the entire combination valve must be replaced, because these units are not repairable.

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Stop Light Switches

The stop light (stop light/speed control) switch and mounting bracket assembly (Figure 25) is attached the brake pedal bracket and is activated by pressing e brake pedal.

The mechanical stop light switch is operated by contact with the brake pedal or with a bracket ached to the pedal.

The hydraulic switch is operated by hydraulic pressure developed in the master cylinder.

In both types, the circuit through the switch open when the brake pedal is released. When the , brakes are applied, the circuit through the switch closes and causes the stop lights to come on.

Hydraulic brake light switches are not commonly used today because they provided one more area for possible fluid leaks.

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Hydraulic System Bleeding

Fluids cannot be compressed, while gases are compressible.

Any air in the brake hydraulic system is compressed as the pressure increases.

This action reduces the amount of force that can be transmitted by the fluid.

This is why it is very important to keep all air out of the hydraulic system.

To do this, air must be bled from brakes.

This procedure is called bleeding the brake system.

Bleeding is a process of forcing fluid through the brake lines and out through a bleeder valve or bleeder screw.

The fluid eliminates any air that might be in the system.

Bleeder screws and valves are fastened to the wheel cylinders or calipers.

The bleeder must be cleaned. A drain hose then is connected from the bleeder to a glass jar (Figure 26).

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CUSTOMER CARE Remind your customers to use only approved brake fluid in a brake system.

Any other lubricant, such as power steering fluid, automatic transmission fluid, or engine oil, which has a petroleum base, must never be used in the brake system. Petroleum-based fluids attack the rubber components in the brake system like the piston cups and seals, and cause them to swell and disintegrate. If you open a brake fluid reservoir and find the diaphragm is swelled out of shape and is larger than it should be, you can be sure some- one has previously put some kind of petroleum-based fluid in it. Such a discovery is costly to the vehicle's owner as all rubber components in the entire brake system will need to be replaced. .

Two types of brake bleeding procedures are used:

1. manual bleeding 2. pressure bleeding.

Always follow the manufacturer's recommendations when bleeding brakes.

The sequence in which bleeding is performed can be critical.

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When bleeding a power brake system, remove the vacuum line from the power unit and plug the unit.

To remove vacuum, the engine must be off. Pump the brake pedal several times-

WARNING! ~ Always use fresh brake fluid when bleeding the system. Do not use fluid that has been drained. Drained fluid may be contaminated and can damage the system. .

Manual Bleeding

A manual bleeding procedure requires two people.

One person operates the, bleeder; the other, the brake pedal. Bleed only one wheel at a time.

WARNING! Be sure the bleeder hose is below the surface of the liquid in the jar at all times. Do not allow the master cylinder to run out of fluid at any time. If these precautions are not followed, air can enter the system, and it must be bled again. The master cylinder cover must be kept in place.

Place the bleeder hose and jar in position. Have a helper pump the brake pedal several times and then hold it down with moderate pressure.

Slowly open the bleeder valve. After fluid/air has stopped flowing, close the bleeder valve.

Have the helper slowly release the pedal. Repeat this procedure until fluid that flows from the bleeder is clear and free of bubbles.

Discard all used brake fluid. Fill the master cylinder reservoir. Check the brakes for proper operation.

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WARNING! Clean the master cylinder and cover before adding fluid. This is important for preventing dirt from entering the reservoir. .

Pressure Bleeding

A pressure bleeding procedure can be done by one person.

Pressure bleeding equipment uses pressurized fluid that flows through a special adapter fitted into the master cylinder (Figure 27).

The use of pressure bleeding equipment varies with different automobiles and different equipment makers.

Always follow the automobile manufacturer's recommendations when using pressure bleeding equipment .

On automobiles with metering valves, the valve must be held open during pressure bleeding.

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Figure 28 shows a special tool used to hold open the metering section of a combination valve.

USING SERVICE MANUALS Consult the service manual to be sure the proper bleeding sequence is followed. If a vehicle requiring a special sequence is bled in the conventional manner, air might be chased throughout the system.

Open the bleeder valves one at a time until clear, air-free fluid is flowing.

Progress from the wheel cylinder farthest from the master cylinder to the cylinder closest.

Do not exceed recommended pressure while bleeding the brakes.

Always release air pressure after bleeding.

Clean and fill the master cylinder after pressure bleeding.

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Be sure to remove the special tool used to hold the metering valve.

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