Brake Proportioning/Combination Valve: Description and Operation

Some vehicles incorporate a hold-off valve, usually in combination with the brake warning switch. This valve holds off hydraulic pressure to the front disc brakes to a given psi range to allow the rear drum brake shoes to overcome the return springs and begin to contact drums. This action helps prevent locking of the front brakes on icy surfaces under light braking conditions. This valve has no effect on front brake pressure during hard stopping.