Fault phenomenon: a Daewoo hope car, brake must step on the brake pedal to the end, in order to brake. Troubleshooting: test the car for a test. This vehicle adopts a rear drum brake system and a vacuum booster pump. It is found in the inspection that the brake pedal travel is very long during braking, and the braking distance is longer after stepping the brake pedal. The brake pedal used in this car is relatively low, but there is nothing wrong with it, and the situation is hard to use now. First of all, check the brake oil cup and find that there is nothing wrong with the oil quality. The brake system was deflated and no gas was found in the brake line. According to conventional experience, it may be that the two piston seals of the main pump are not good enough to cause the oil to pass through the oil seal and return to the oil cup. Remove the total pump and check, no problem is found. Since the main pump is not easy to disassemble, it is still difficult to test after exchanging with other cars, indicating that there is no problem with the main pump. The fault may be caused by the oil leakage of the brake sub pump, but the brake oil is not less. Holding a line of hope, dismantling the four wheel, checking the various brake sub pumps, the result found that two front brake sub pump skin bowl has the oil seepage mouth, but the oil is not enough to appear above the above fault, moreover, when braking, the oil is sealed in the system. In order to prevent the leakage of some unexpected places, separate the oil pipe connection bolts of each sub pump, make the special gasket to block the tubing, and then test the 4 sub pumps in turn. Finally, it was found that after the left front pump was blocked, the brake system quickly returned to normal, and the pedal was no longer trampled to the end. The problem was on the left front brake sub pump. After dismantling the sub pump, it is found that the slideway of the fixed pump has been rusted with the sub pump, which leads to the free movement of the sub pump, which makes the clearance adjustment of the left front brake disc become impossible, so the left front brake clearance is great. When stepping on the brake pedal, a large part of the brake fluid enters the left front pump. When the effective stroke of the pedal is constant, that is, the total brake fluid supply is constant, and the gap between the four wheel brake disc and the brake disc (or drum) is increased, so the actual oil flow on the brake is reduced relatively, and the actual braking pressure is also reduced. After thoroughly removing the corroded parts, changing the oil, releasing the gas and testing the car, the faults of the brake system are completely eliminated.