The Brake System:
Trouble shooting Air-Hydraulic Brake Systems
After completing an inspection and road test of a vehicle braking system, you should have a pretty good idea of what the system is doing wrong. The next thing to do is to find out what is making the system do something wrong. This is known as troubleshooting and consists of isolating or locating the part or parts that are causing the trouble. We will refer to the major trouble as a malfunction and discuss several things that could cause each malfunction as well as what should be done to correct the trouble.
Insufficient Brakes
You will sometimes find a braking system that will not press the brake linings against the drums hard enough to stop the vehicle in the proper distance.
Check the wheel brake lining condition and adjustment. If the linings are worn badly, they should be replaced. If they are too far from the brake drums, they should be adjusted.
Check the air pressure system to ensure there is a minimum of 80 to 105 PSI. Correct any leaks and replace components that are not working.
Check the air-hydraulic cylinder for proper operation. Certain tests (which we will discuss later) can be made to determine if this component is doing its job.
Brakes Apply Too Slowly
If you depress the brake pedal and the braking action does not occur immediately, it indicates there is a bind or weakness some place.
First, ensure you have sufficient air pressure and there is no water (condensation) in the compressed air system.
Check all air and hydraulic lines to ensure they are not kinked or bent to a point where they will restrict the flow of air or hydraulic fluid.
Check the brake shoes at their anchors. They should move freely and not bind.
Brakes Dragging
This malfunction is noticeable when you are road-testing a vehicle. The vehicle holds back when you are trying to accelerate, and when you disengage the clutch, the truck will not coast freely.
First, check the brake pedal free play adjustment to ensure you have the proper clearance. If the pedal does not allow the master cylinder piston and primary cup to clear the compensating port, the brake fluid cannot come back to the master cylinder reservoir from the wheel cylinders.
Next, check the brake adjustment of each wheel to ensure there is enough clearance when the brakes are released.
Restricted air and hydraulic lines can also prevent the brakes from releasing.
Ensure the brake shoes are not sticking on the anchor pins.
Check the master cylinder for operation. If some foreign material, such as oil or dirt, is mixed with the brake fluid in the reservoir, it can plug the ports or warp the rubber cups. This will prevent the fluid from returning to the reservoir.
Sticking pistons in the wheel cylinders can also cause dragging brakes. If the pistons or cylinders are corroded, the cylinder assemblies should be replaced.
As a last resort, check the operation of the air-hydraulic cylinder. It is possible that the air valve is not allowing the compressed air in the cylinder to escape. If tests indicate improper operation, the air-hydraulic cylinder should be replaced.
Brakes Grab
This malfunction is most noticeable when you first apply the brakes. As soon as the brakes are applied, the wheels tend to lock and the tires slide on the road. Sometimes only one wheel will grab, while at other times more than one will grab.
Grease, fluid, or grit on the brake lining is the primary cause of this fault. With this condition the lining must be replaced.
If the brake shoes are binding on the anchors, they will snap out to the drums when enough pressure is applied.
Loose spring D-bolts or improperly adjusted wheel bearings can cause the position of the wheel to shift when the brakes are applied.
Loose lining on brake shoes and weak or broken brake shoe return springs will allow the brakes to grab.
Brake drums that are out of round, scored, or cracked can also cause brakes to lock up.
Noisy Brakes
Squealing or grinding noises when the brakes are applied are not normal.
Glazed lining that makes the brakes squeal should be replaced.
Scored brake drums and linings that are worn down to the rivets or shoes will cause a grinding noise.
If the brake shoes, backing plates, or anchor pins are bent or warped, the brakes will make an unusual sound when applied. |