System diagram of automobile brake system architecture
The braking process is divided into linear braking and non-linear braking. The so-called "soft braking" is non-linear braking, that is to say, the strength of the foot on the brake pedal is not proportional to the actual braking effect, and it needs to be stepped deeper to achieve braking. influences. There are several reasons for soft braking:
(1) Separate cylinder oil leakage and pressure relief;
(2) Oil leakage and pressure relief of the master cylinder;
(3) The brake fluid is impure or contaminated, has a low boiling point, and generates bubbles during braking;
(4) There is air in the braking system;
(5) The brake clearance is too large.
For the two reasons of the first and second points, the brake pump leakage and pressure relief, the specific diagnostic method is to step on the brake, step on it to the end, and then continue to step on with the same force. If the pedal is pedaled, it will slowly decrease. , It means that the pressure of the pump is relieved, which is usually caused by poor sealing of the master cylinder and the sub-cylinder (damaged sealing ring), which is rarely seen in new cars.
The third point is that the brake oil is impure or contaminated, and the boiling point check is mainly performed by the boiling point meter of the detection instrument. The dry boiling point of DOT4 is required to be more than 230 degrees, and the wet boiling point is required to be more than 155 degrees. This problem needs to be focused on the production site.
The fourth point is that there is air in the brake system. Generally, the air in the brake system of the vehicle (especially the left and right rear wheel brake pipes) is not sufficiently extracted, and there is more air in the pipes. After the brake fluid is filled The remaining air is compressed to the ends of the four wheels, resulting in no brake fluid or insufficient brake fluid at the ends, so the brake "soft brake" phenomenon appears. To solve the problem of “unclean” air in the automobile brake system, first check whether the ABS/ESP pump valve module of the automobile is opened during the vacuuming and filling process of the equipment (the brake valve will open at intervals during the vacuuming process, And make a "click" sound, the brake pump will make a "buzzing" sound during the filling process); In addition, the vacuum pumping capacity of the brake fluid filling equipment, that is, use a digital vacuum gauge to check the brake fluid filling The vacuuming capacity of the equipment, if the vacuum value can reach 3mbar or less, it is confirmed that the vacuum capacity of the equipment is sufficient and the equipment vacuum system is well sealed. Otherwise, the equipment vacuum capacity is insufficient or the equipment vacuum system is not tightly sealed. This problem needs to be verified and solved by production site equipment maintenance personnel and process personnel.
The fifth point is that the brake free gap is too large. This is a description of the feeling of brake control. This gap refers to the gap between the end of the master cylinder push rod and the master cylinder piston seat (usually 1.5~2.5mm). The free stroke of the moving pedal can be adjusted by manually rotating the length of the push rod. Each new model has strict technical standards for the free stroke of the brake pedal, and new vehicles rarely have such problems.