Abstract : This paper briefly expounds the causes of hydraulic brake system failures such as brake efficiency, brake failure and deviation, as well as the diagnosis and solution measures.
The brake failure caused by the hydraulic brake system is often encountered in the actual work, the following brief analysis of several major faults that are prone to occur, for the reader reference.
1 brake bite
Phenomenon: When you step on the brake pedal, you feel high and hard or do not have a free trip. The car is difficult to start or hard to drive.
the reason:
(1) Difficulty in adding pedal shaft rust.
(2) The return spring of the main pump piston is too soft and broken, and the swelling of the bowl blocks the return oil hole or the return oil hole is blocked by the dirt.
(3) The main pump bowl, apron swells, or the piston deforms or becomes contaminated.
(4) The brake shoe can rotate freely above and below the support pin.
(5) Brake shoe friction plate and brake drum clearance is too small.
(6) The brake shoe return spring is too soft and broken.
(7) The brake fluid is too dirty and the viscosity is too high, making it difficult to return oil.
(8) The pump bowl is swollen, the piston is deformed or there is dirt stuck.
(9) The brake pipe is concave and blocked, which makes the oil return poor.
(10) The brake pedal does not have a free stroke or its return spring is disengaged, broken or too soft.
diagnosis:
After the brake pedal is released, all or individual wheels still have a braking effect, which means that the brakes are biting. Braking occurs during driving. If all the brake drums are overheated, the master pump is faulty. If the individual brake drums are overheated, the wheel brakes are not working properly.
If the fault is in the main pump, check the free travel of the brake pedal first. If there is no free stroke, generally the clearance between the master cylinder push rod and the piston is too small or there is no clearance. If the free stroke is normal, remove the plug of the main pump reservoir and lift the brake pedal to observe the oil return. If it does not return oil, block the return oil hole. If the return oil is slow, check if the brake fluid is too dirty and the viscosity is too high. If the brake fluid is pure, the total pump cups and aprons may swell or their return springs may be too soft and should be checked by the main pump.
If the fault occurs on the individual wheel brakes, set the wheel and loosen the pump bleeder screw. If the brake fluid is quickly sprayed and the wheels turn freely, it means that the brake pipe is blocked and the cylinder is not Can return oil. If the wheel is still rotating, check whether the clearance between the brake shoe friction plate and the brake drum is too small. If all of the above are normal, check the condition of the bowl pump and the brake return spring.
2 Bad braking performance
Phenomenon: The braking deceleration is small and the braking distance is long when the vehicle is braking while driving.
the reason:
(1) Air in the brake pipe is infiltrated.
(2) The brake is malfunctioning.
(3) The sub-pump failed.
(4) The main pump is defective.
diagnosis:
The reason why the hydraulic brake system produces poor braking performance is generally based on the brake pedal stroke (commonly known as high and low), the soft and hard feeling when the brake pedal is depressed, the stability after depressing the brake pedal, and the pedal when braking. Increase the height to judge.
(1) When the brake is normal, the pedal height is too low and the braking performance is poor. If two or more feet are braked continuously, the pedal height will increase with this and the braking performance will improve, which means that the clearance between the brake drum and the friction plate or the master cylinder piston and the push rod is too large.
(2) When a few feet are braked continuously, the height of the pedal is low and soft, which means that the vent hole of the reservoir plug in the total oil inlet hole is blocked.
(3) When the brakes are continuously braked, the pedal height is still too low, and after the second foot is braked, it feels that the master pump piston is not returned, and the brake pedal is pressed by the master pump push rod and the piston. It is the total pump cup rupture or a few consecutive feet, the return spring is too soft.
(4) A few feet in succession, the pedals were stepped to the end, and there was no reaction force on the pedals, indicating that the brake fluid in the main pump's fluid chamber was seriously depleted.
(5) The height of the pedal is slightly increased and there is a sense of elasticity when the brakes are continuously braked. This indicates that air has penetrated into the brake pipe.
(6) When the brake is maintained, if the height of the pedal is slowly or rapidly decreased, it indicates that the brake pipe is broken somewhere, the joint is tightly closed, or the seal of the bowl is poor, the return spring is too soft or broken, or the total pump skin The bowl and the apron are poorly sealed, and the oil return valve and the oil outlet valve are defective. The brake pedal can be depressed first to observe the presence of brake fluid leakage. If the outside is normal, check the sub-pump or master pump for failure.
(7) When one or two feet are braked, the pedal height is appropriate, but too hard braking performance is poor. It should be checked whether the gap between each wheel friction plate and the drum is too small and high speed is inappropriate. If the clearance is normal, check the surface condition of the drum and the friction plate. If normal, check whether the brake shoe spring is too hard, whether the main pump or the sub-pump cup is swollen, and whether the piston and the cylinder wall are loose. If both are normal, you should check whether the brake hose is aging.
3 Abrupt brake failure
Phenomenon: When the car is moving, one or more feet are braked. The brake pedals are stepped to the end and the brakes suddenly fail.
the reason:
(1) The brake pipe is severely broken or the joint is out of joint.
(2) Breaking or overturning of the pump bowl.
(3) The main pump bowl is damaged or overturned.
(4) There is no brake fluid in the main pump.
diagnosis:
In the event of failure of the brake, it should be stopped immediately. First observe the presence of leaking brake fluid. Such as the brake master cylinder push rod at the brake fluid at the Department; such as the brake master cylinder push rod dust jacket at the leakage of serious fluid, mostly a total pump bowl overturned or severely punished damage; such as a wheel system There is a lot of brake fluid on the edge of the drum, which indicates that the bowl of the wheel cylinder was overturned or seriously damaged. Leakage of brake fluid in the pipeline is generally obvious. If there is no leakage of brake fluid, check whether the brake fluid in the reservoir of the master cylinder is sufficient.
4 Brake deviation (unilateral)
Phenomenon: When the car is braking, it is skewed to one side.
the reason:
(1) The deformation of the frame, the displacement of the front axle, the unsuitability of the front toe, the looseness of the steering mechanism and the elasticity of the two front leaf springs.
(2) The gap between the two front wheel brake drums and friction plates is different, the contact area of ​​the two front wheel friction plates is too different, the quality of the two front wheel grinding plates is different, and the inner diameter of the two front wheel brake drums is too different. Front wheel brake shoes return spring springs are not equal.
(3) The degree of wear of the eccentric sleeves of the two front wheel brake shoe support pins is not uniform.
(4) The two rear wheels have the above-mentioned first three faults.
(5) The piston and cylinder of one side of the front wheel have excessive friction.
The wheel cylinders have air, hoses are aging or the cylinders of the sub-pumps are poor, or the brake drums on one side of the front wheel are out of round. The tire pressures of the two front tires are inconsistent, and the oil, dirt, water, hardening, and rivets of the front-wheel friction plates are exposed.
diagnosis:
During the inspection, the road test brake was used first, and the wheels with poor braking performance were checked and repaired according to the tires. The wheels with short or no trailing marks are poor braking performance. You can check whether the wheel brake line leaks and whether the tire pressure is sufficient. If normal, the gap between the disc and the brake drum can be rubbed at high speed. If it is still invalid, check whether the pump penetrates into the air. If there is no air infiltration, remove the brake drum and check each part of the brake one by one. If it is normal, it means that the fault is not in the brake system. The technical condition of the frame or front axle and the steering mechanism should be checked. If it is more convenient to check the brake test bench, which wheel braking force is small, it is a bad wheel.
As for the braking system of Santana vehicles, it is also a hydraulic brake. However, they are all caliper brake mechanisms. If a fault occurs, check the free travel of the pedal, the brake fluid level of the brake fluid reservoir, the thickness of the brake pads, and check the brake pressure of the brake pressure regulator. Combined request.
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