Q: Would you please explain residual valves?
A: A residual pressure valve is a special type of one-way check valve. They are used to keep a small amount of pressure in the brake lines. This helps the brakes engage more quickly and reduces pedal travel.
There are two basic types of residual valves — 10 psi and 2 psi.
How do you identify residual valves?
In the aftermarket, 10 psi valves are usually red. The 2 psi valves are usually blue. However, other colors are available. The valves should be located on or near the master cylinder.
In OEM applications, the residual pressure valve is usually built into the master cylinder. In this case, an external valve is not required.
When are residual valves used?
A 10 psi residual valve is used with drum brakes. The valve holds 10 psi in the brake lines going to the wheel cylinders. This keeps pressure against the return springs inside the drum. This means the brakes engage faster with less pedal travel.
Most disc brake systems do not require a residual valve. However, a 2 psi residual valve is used when the master cylinder is lower than the calipers.
When the master cylinder is mounted on the firewall, it is usually higher than the calipers. Gravity keeps the fluid from flowing backward. In this case, a residual valve is not required.
In some vehicles, the master cylinder is relocated to a lower position. This could be under the floor or on the frame rail. In this design, a residual valve is required to prevent fluid back-flow.
Without a residual valve, the pedal could feel “spongy.” It would also take more pedal travel to move enough fluid to engage the brakes.
- When converting from drum to disc brakes, you should remove the 10 psi valve.
- The constant line pressure causes the brakes to drag.
- This will cause overheating and premature wear.