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Failure phenomenon: a Chery 2 car, the cumulative driving about 14 thousand km normal stop after second days to step on the brake pedal, feeling very soft, can step on the end, there is no brake failure phenomenon. Inspection and analysis: the owner of the car was informed that the above fault occurred after the vehicle was stopped for a night, and the braking performance of the vehicle was normal when it was parked. But after stepping on the brake pedal several times, the brake pedal hardens and the braking effect returns to normal. The conductivity of the vacuum tube (internal check valve) is detected, only one end can be turned on, and the other end is normal. Start the engine and let idle speed change to 5min before testing. Stepping on the brake pedal, there is a strong sense of power, the vacuum booster has no air leakage; after stalling, the brake pedal is continuously stepped on 4 times, and the position of the brake pedal is gradually rising and the foot feels hard. After the engine is started again, the brake pedal falls slowly and falls to normal position, then no longer descends. After the engine is extinguished, the brake pedal does not rebound. This indicates that the vacuum booster shop is working normally without malfunction. Because the braking performance affects the driving safety, the fault is seriously analyzed. It is considered that the fault is caused by the contraction of the inner seal ring after the cooling of the main cylinder after one night cooling, and the failure is on the main cylinder of the brake. Elimination method: replace the main cylinder of the brake.