How to Test and Diagnose Brake Valve Failures

Whether you are working on an air brake system or just looking for a few tips, the importance of brake valving is an underappreciated component of the entire system. Brake valving is a critical part of the system, and failures can affect the proper operation of the system. It is therefore important to know how to test and diagnose brake valving.

There are three types of brake valving: "smart" proportioning valves, "load sensing proportioning valves," and "relay valves." Smart valves work by detecting the ride height of a vehicle and responding accordingly. Load sensing proportioning valves work with the rear suspension to maintain the proper ride height. Relay valves use air from the manual trailer valve or dual control foot valve. The "crack" pressure that a relay valve needs to work is determined by the size of the brakes and the load on the axle.

Whether you are working on an air brake, hydraulic, or spring brake system, you should consider how your valves work when performing a maintenance inspection. If you are not sure how to test or adjust a valve, there are many online resources that will help you with this task.

The first thing to check is the flow of rear brake fluid. Each rear brake is on a separate circuit, so if the bleeder is closed on one side of the vehicle, the other should be open. If the flow of the rear brakes is different, then the valves are in need of adjustment.

A proportioning valve works by limiting the pressure on the rear brakes. This prevents rear wheel lockup during panic braking situations. If a proportioning valve fails, it will usually cause the rear brakes to work longer than they should. This will cause the brakes to work insufficiently and may also cause the trailer to push or lockup.

It is also important to note that most vehicles will use rear drum brakes. If a vehicle has disc brakes, the disc brakes will work in conjunction with the drum brakes. The combination of the two brakes will give a balanced braking response.

If you have a brake system that uses air valving, you must know how to test the brakes and replace the valve if necessary. Brake valves can fail due to design or location. If you are not sure whether your brakes need to be replaced, it is a good idea to have a technician perform a diagnosis.

Another area that is often overlooked when working on a brake system is the metering valve. Most manufacturers do not publish the pressures that their brakes require. Most of the time, metering valves fail because of external problems. The best way to diagnose a metering valve is to use a pressure gauge. If the pressure is not correct, the valve will be stuck in the partially open position. This may cause the brakes to work improperly, not give a good stopping power, or cause the master cylinder to shut down when the vehicle is applied.