Tuesday, February 7, 2012

How to replace Brake Power Booster?

Before replacing brake power booster,first test the brake power booster and confirm its fault.

The brake power booster is a device installed in a hydraulic brake system that reduces pedal effort and affords better braking.
This is how it looks :----

Now follow this procedure to test the brake power booster.

Internal air leaks in the booster can prevent proper booster operation. A booster with internal leaks should be replaced or rebuilt.

An internal leakage check, also called an air-tightness test, is made by running the engine for one or two minutes then shutting it off. Then apply the brake with normal pedal pressure and hold it at least thirty seconds. If the pedal remains steady, the booster is probably good. If the pedal slowly rises, the booster has an internal leak.

Booster internal leaks can also be checked by fabricating a simple tool. Choose a glass jar that has a tight-fitting lid and attach two 3/8-inch (9.5 mm) OD tubes to the lid. One tube should be long enough to reach almost the bottom of the jar, and the other should end just inside the lid. Fill the jar about half full with water and connect a short tube to the booster supply hose from the intake manifold. Then use a short piece of hose to connect the long tube to the booster inlet. Start the engine and watch for air bubbles in the jar as you operate the brake pedal a few times. Airflow from the booster to the manifold will cause air bubbles to pass through the water in the jar. As the pedal is released, there should be a large flow of bubbles, bat after a moment this flow should stop. With the brake pedal released or applied, there should be no flow. A constant flow of bubbles with the brake applied indicates a leaky piston diaphragm. A constant flow with the brakes released indicates a leaky control valve.

A vacuum booster can also be checked for internal leakage with a vacuum pump. Remove the vacuum hose from the check valve and connect the vacuum pump directly to the check valve or inlet fitting. With the pedal released, you should be able to draw a 17- to 20-inch vacuum, and this vacuum reading should hold steady for several minutes. A leak in the control valve or booster chamber is indicated if the drops. Next, apply the brakes with moderate pressure on the pedal. An immediate drop in the reading should occur as the pedal moves. Draw the vacuum back to 17 to 20 inches and observe the reading to make sure it does not drop more than 2 inches in the next thirty seconds. A leaky diaphragm, control valve, or vacuum chamber is indicated if it does drop.

Once the power booster is test faulty or weak,then follow the procedure mentioned below to replace it.
The details are as follows :-----

Most shops do not rebuild power brake boosters. Rebuilt units are available for most cars and trucks. When replacing a power booster, it is important that the pushrod depth is correct. There are special tools and different methods specified for adjusting this.
Tools for checking power booster pushrod depth.

Complete rebuilt units are available that include a rebuilt master cylinder assembled to them.
A faulty booster can be removed for rebuilding or replacement with a new or rebuilt unit. This can be relatively easy operation depending on the location of the bolts and nuts securing the booster to the car's bulkhead.
NOTE:--------On some vehicles, the brake pedal will drop and activate the brake lights when the booster is removed, and the extended operation of the center brake light can overheat the rear window or all of the lights can discharge a battery. Check the light operation after booster removal, and if the lights are lit, prop up the pedal to turn them off.
To remove the power booster:
  • On Hydro-boost and Powermaster units, first remove the pressure in the accumulator. With the engine off, apply and release the brake several times until you feel no change in the hardness of the pedal. On Powermaster units, ten to twenty pedal applications are recommended.
  • Disconnect the master cylinder from the booster. On many cars, the brake lines are long and flexible enough to allow the master cylinder to be moved away from the booster without disconnecting them. If not, remove the master cylinder.
On some vehicles, the brake pedal will drop and activate the brake lights when the booster is removed, and the extended operation of the center brake light can overheat the rear window or all of the lights can discharge a battery. Check the light operation after booster removal, and if the lights are lit, prop up the pedal to turn them off.
On Powermaster units, the brake lines must be disconnected because the booster and the master cylinder are combined.
  • Disconnect the booster power supply.
  • On a vacuum booster, disconnect the vacuum line.
  • On a Hydro-boost unit, disconnect the power steering pump and steering gear fluid lines.
  • On a Powermaster unit, disconnect the electric connectors.
  • Disconnect the pushrod from the brake pedal.
  • Remove the nuts or bolts securing the booster to the bulkhead and remove the booster.
Booster installation is essentially the reverse of the removal procedure. During installation, check the master cylinder pushrod adjustment. A pushrod that is too long can position the primary piston too deep in the cylinder bore. The primary cup will then cover the compensating port and cause brake drag. A short pushrod can cause a low brake pedal. Three different procedures can be used to check this adjustment: the gauge method, the air method, and the fluid swirl method. Many booster use an adjustable pushrod which can be lengthened or shortened as necessary.
The gauge method is the quickest. A gauge can be easily shop-made from cardboard or thin sheet metal if the dimensions are available. It is normally a two-step, "go-no go" gauge. It is placed in position with the pushrod, touching the short step of the gauge but not the long one.
For the fluid swirl method, after the booster and master cylinder are mounted, have an assistant apply the brake pedal as you observe the fluid in the primary reservoir section. The reservoir should be about half full of fluid, and a clean plastic film should be placed over the reservoir to contain the fluid. A slight swirl should be seen in the fluid as the brakes are applied and also during release. No swirl on application indicates that the pushrod is too long. A larger-than-normal swirl on application indicates that the pushrod is too short. If this occurs, loosen the master cylinder mounting bolts about one -fourth of an inch and repeat the test. If a normal swirl is now present, the pushrod is too long.
With the mastery cylinder mounted on the booster, and the brake line is disconnected from the primary outlet part. then clean, low-pressure compressed air is blown into the outlet port. if the air passes through the compensating port into the reservoir, the pushrod is not too long. note that this method will require bleeding of the primary section of the master cylinder.
To replace a booster:
  • Place the booster in position, install the nuts or bolts securing it, and tighten them to the correct torque.
A Powermaster unit must be bench-bled in the master cylinder section before installation.
  • Reconnect the pushrod to the brake pedal.
  • Remount the master cylinder on the booster and tighten the nuts or bolts to the correct torque.
  • Connect the booster's power supply: On a vacuum boosters, reconnect the vacuum hose. On Hydro-boost units reconnect the lines to the power steering pump and gear, tighten them to the correct torque, and bleed the air out of the lines. To bleed the lines:
    • Fill the power steering pump reservoir.
    • Crank the engine for several seconds but do not start it.
    • Check the power steering reservoir and refill it as necessary.
    • Start the engine and slowly turn the steering wheel from stop to stop twice.
    • Do not hold the steering wheel against the stops.
    • Stop the engine and apply and release the brake pedal several times to bleed the pressure out of the accumulator.
    • Check the reservoir and refill it as necessary.
  • On Powermaster units, reconnect the electric connector, fill all three reservoir sections with brake fluid to the correct level, and turn on the ignition. The pump should run and shut off within twenty seconds. If it is still running after twenty seconds, shut off the ignition and refer to the diagnosis chart for the solution to this problem. Power master pumping pressures are tested using a pressure gauge.
  • Check the master cylinder reservoir to refill it if necessary. Start the engine and check the booster operation. Note that if the master cylinder has been completely removed, it will be necessary to bleed the brake hydraulic system.
On Powermaster units, the pump section of the reservoir will have the correct fluid level only when the accumulator is discharged. Filling the reservoir after the accumulator is charged will cause an overflow and spillage when the accumulator discharges.

This details will help.

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