Hydraulic Brake Valves
Brake valves are an important part of the brake system because the valves are used to control the flow of the brake fluid. If it was not for the brake valves, then all four wheels would get the same brake pressure creating a hazardous braking situation.
There are several different types of valves that can be found on a vehicle's brake system including: residual check valves, proportioning valves, height-sensing proportioning valves, metering valves and combination valves.
Residual check valves can be found on some drum brake systems so that roughly 5 to 12 PSI of pressure stays in these systems. This helps the drum brake application so that the drum brakes do not have to overcome spring pressure. This check valve is located in the master cylinder at the outlet to the drum brakes.
The proportioning valves primary purpose it to control brake fluid pressure to the rear brakes. The pressure to the rear brakes is controlled by pressure sent from the master cylinder to one side of the spring-loaded piston that is in the valve, while the other side of the piston reacts to the rear brake pressure. The proportioning valve does not function during light and moderate braking applications, because there is insufficient weight transfer and the chance that the rear wheels would lock up during these conditions is slim to none. On the other hand, during heavier braking applications, the brake fluid pressure is greater, so the proportioning valve will then limit the pressure going to the rear brakes—resulting in increased pressure to the front brakes.
The height-sensing proportional valve works in the same manner as the regular proportioning valve, except the height-sensing proportional valves vary the amount of pressure to the rear brakes, based upon the relative ride height. Also, certain vehicle manufacturers warn against lifting a vehicle that has a height sensing proportioning valve, because the driver could experience reduced stopping distances. This is because, when you lift a vehicle, the lever on the height sensing proportioning valve will be raised all the way to the top, causing lower to the rear brakes all the time—even when the vehicle is carrying a heavier load. During heavier loads, the rear brakes need greater braking pressure, but if the lever on the valve is all the way to the top, there is actually less pressure than can accommodate the load.
Metering valves are used on all front-disc, rear-drum-type brake systems. The metering valve holds off front-disc brake pressure until the rear brakes reach close to 125 PSI. This delay to the front brakes is important, because the rear-drum brakes have to overcome return spring pressure so the front and the rear brakes can stop at the same time for even stopping.
A combination valve either combines the function of a proportioning valve with a pressure-differential switch (alerts drive of loss of brake pressure), or a combination valve can include a pressure-differential switch combined with a metering valve.
For complete details as to which valve or valves your vehicle may have and how to test the valves, refer to our brake inspection page. After determining the valve that has failed, the following steps will assist in the replacement of the valves.
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