Examination
The brake booster increases the force applied to the brake pedal, thereby making the vehicle easier to control. The operating principle of the vacuum brake booster is based on the use of vacuum in the intake manifold. If the vacuum booster fails, the performance of the brake system is not affected, but significantly more force must be applied to press the brake pedal.
Press the brake pedal several times with the engine off to create a uniform pressure in the vacuum booster cavities, close to atmospheric pressure. At the same time, by the force applied to the pedal, determine whether the valve body is jammed. Stop the brake pedal in the middle of its travel and start the engine. If the vacuum booster is working properly, the brake pedal should go down after starting the engine. If the pedal does not move, check the vacuum hose tip fastening, the tip flange connection in the booster, the condition of the hose and fitting.
Wait two minutes and turn off the engine. Press the brake pedal. With each subsequent press, the pedal should become harder and the pedal travel should decrease.
Removal
Disconnect the vacuum hose connecting the inlet manifold and the brake booster. Be careful not to damage the vacuum hose when removing it from the vacuum booster valve fitting.
Remove the brake master cylinder. On pre-2001 models, remove the air intake duct and fuse/relay box.
Remove the lower trim panel from the left side of the instrument panel.
Remove the brake pedal return spring.
Figure 11.39. Brake booster mounting: A – pin; B – nuts
Disconnect the brake booster pushrod from the top of the brake pedal by removing the cotter pin and pulling out the pin (figure 11.39).
From inside the vehicle, unscrew the four nuts securing the brake booster to the engine compartment bulkhead and remove the brake booster from the engine compartment.
Installation
Installation is carried out in the reverse order of removal, taking into account the following.
If a new vacuum booster is being installed, check the tappet clearance as follows:
Figure 11.40. Measuring the brake booster tappet protrusion (dimension A)
- measure the distance the brake booster pushrod protrudes from the master cylinder mating surface (figure 11.40), and write it down as size A;
- measure the distance from the flange end to the mating surface of the master cylinder and record it as dimension B;
- measure the distance from the flange to the master cylinder piston and record it as dimension C;
Figure 11.41. Clearance between the pusher (2) of the brake booster and the piston (1) of the brake master cylinder: X = 0 mm
- subtract dimension B from dimension A, then subtract dimension C from the resulting value (A - B) to get gap X (figure 11.41) pusher 2, which should be equal to 0. If the gap value is negative, i.e. the vacuum booster pusher presses on piston 1 of the master brake cylinder, then constant braking of the wheels will occur. If there is a gap, the brake pedal travel will increase when braking.
If the gap size does not correspond to the required value, then adjust it by changing the length of the pusher. Use pliers to hold the brake booster pushrod from turning and use a wrench to screw the pushrod head in or out.
After installing the master cylinder and connecting the brake lines to it, adjust the brake pedal height and bleed the hydraulic brake system.