The proportioning valve adjusts braking force to allow for load transfer. It can be pressure-sensitive, or load-sensitive.
The pressure-sensitive valve can be in the master cylinder, or in a separate unit in the rear brake circuit.
The load-sensitive type can be in the body or the axle, where it can respond to load changes, and change the braking effort as needed.
Master cylinder applications usually combine the proportioning valve with a pressure differential switch.
In normal braking, the poppet piston is held in a relaxed position by a large pressure spring. The poppet valve is held against its retainer by a light return spring, and fluid passes freely through the valve to the rear brakes.
In heavy braking, master cylinder pressure can reach a valve’s crack-point. The pressure applied to the 2 different areas of the poppet piston creates unequal forces. That moves the poppet piston against the large pressure spring. This action holds the conical section of the valve against the seat, which limits the pressure increase to the rear brakes.
As greater pedal force increases pressure in the master cylinder, fluid pressure rises on the smaller end of the piston. This combines with the force of the pressure spring to overcome the lower pressure now on the larger end. This forces the piston back, clear of the poppet valve.
The increased pressure now acts on the larger end of the poppet piston and again forces the piston forward to contact the valve.
When the pedal is released, the pressure of the rear brake fluid unseats the poppet valve, letting fluid return to the master cylinder. The pressure spring now returns the poppet piston to its relaxed position.
Should the front brake system fail, the warning lamp spool moves forward, taking the poppet valve with it. Pressure in the rear brakes rises and the piston moves forward but it can’t seal on the valve.
Should the rear brake system fail, the warning lamp spool will move backwards to activate the warning light. The proportioning valve doesn’t operate in this situation.
On a diagonally-divided system, the pressure-sensitive proportioning valve is usually located away from the master cylinder. There is one for each circuit. They each operate in a similar way to the pressure-sensitive proportioning valve located in the master cylinder, but without the pressure-differential warning light circuit.
The load-sensing proportioning valve is usually located in the rear brake circuit, on the chassis. A diagonally-split system may have 2 load-sensing proportioning valves, one for each brake. The unit is mounted on the chassis, around the rear suspension.