Brake Proportioning Valve
Proportioning valves enable balanced braking by reducing hydraulic pressure to the rear brake units. This reduction in pressure helps prevent rear wheel lockup. During braking, most of the vehicle's weight is shifted to the front leaving the rears with little traction. Many of these valves are integral and part of the combination valve found on front disc, rear drum brake systems.
All height sensing proportioning valves utilized the vehicles ride height to regulate hydraulic pressure. The valve must be installed at vehicle ride height unless vehicle manufacturer's specifications dictate otherwise. These brake valves reduce pressure to the rear brake circuit if there is a sudden stop, preventing wheel lockup. If the valve is faulty or if the brake shoes are contaminated and grabbing, the rear wheels will lock up during hard braking.
If the vehicle is raised higher than the specified with springs or a lift kit, increased ride height and reduced rear hydraulic pressure could have an adverse effect on brake performance. This is because the linkage that manually controls the valve is connected between the axle and the frame. As the axle and frame move further apart, hydraulic pressure is reduced allowing the wheel to spin more freely. This type of valve is popular with manufacturers of pickup trucks.
Spring loaded piston type proportioning valves are either attached to the master cylinder or in line to the brake circuits. These systems cycle back and forth to maintain the required pressure on the rear brakes. They have spring loaded pistons with different diameters at the ends. The size of the piston dictates the pressure reduction. This valve is controlled by hydraulic pressure applied to a spring loaded piston.