A Proportioning valve is required on vehicles
that have disc brakes on the front wheels and drum brakes on the rear wheels.
Disc brake pads are normally in contact with the disc, while the drum brake
shoes are normally not in contact with the drum. If the pressure was not
proportioned the disc brakes would engage before the drum brakes when you
depress the brake pedal.
The Proportioning valve compensates for
this, allowing the drum brakes to engage first before the disc brakes. The
Proportioning valve does not allow any pressure to the disc brakes until
a pre-determined pressure has been reached. The pre-determined pressure
is low when compared to the maximum pressure in the braking system, this
allows the drum brakes to engage before the disc brakes engage. Having the rear brakes engage first provides the
control and stability needed to stop your vehicle safety.
The proportioning valve reduces the pressure
to the rear brakes. Whichever type of brakes your vehicle has, the rear
brakes require less pressure than the front brakes.
If equal braking force were applied to all
four wheels during a stop, the rear wheels would lock-up before the front
wheels. The proportioning valve only lets a portion of the amount of pressure
to the rear wheels thus preventing rear wheel lock-up.