You will need a ruler.
1. Start the engine, let it idle and depress the brake pedal vigorously several times.
2. Pull the brake pedal up with your hand until it stops.
3. Release the pedal and measure the distance from the pedal pad to the floor.
Note. The ruler must be installed perpendicular to the plane of the floor.
4. Vigorously press the brake pedal to the stop.
5. Release the pedal and re-measure the distance from the pedal pad to the free floor. The pedal should return to its original position by the distance from the pad of the pedal pad to the floor, measured in step 3.
6. Check the free play of the brake pedal, for which move the pedal by hand until the pedal stops moving without resistance. Determine the value of the free play on the ruler, it should be 1-3 mm. The discrepancy between the free play and the specified value can be caused by the following reasons:
- increased play in the connection of the pusher of the vacuum booster and the pedal. This may be due to wear of the pusher retainer;
- jamming of pistons in the main brake cylinder;
- jamming of the plunger in the brake booster.
7. If the distance from the pedal pad to the floor when remeasured (item 5) less than the value measured in paragraph 2, this indicates the pedal is stuck on the axle, the pistons in the working and master cylinders are jammed, or the compensation holes in the master brake cylinder are clogged.
Note. Compensation holes in the main brake cylinder can be blocked by cuffs that are swollen due to the ingress of petroleum products into the system, as well as due to the use of low-quality brake fluid.
8. Stopping the pedal when pressed almost at the very floor indicates the presence of air in the hydraulic brakes or the wear limit of the brake pads and discs. If there is air in the system, the pedal stops farther from the floor with a few repeated presses («pumped up»).
Attention! If the pedal is pressed to the stop and held in this position gradually moves to the floor, fluid has leaked from the hydraulic drive or the brake master cylinder is faulty. These malfunctions are very dangerous, so find the leak immediately and fix it or replace the brake master cylinder!
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