Brake Proportioning/Combination Valve: Description and Operation

STRUCTURE OF P & DIFFERENTIAL VALVE
The P & Differential valve consists of a proportioning valve and differential switch.





PURPOSE AND OPERATION OF PROPORTIONING VALVE
The proportioning valve regulates the brake fluid pressure in the rear brake line, which is the same both on the master cylinder side and wheel cylinder side up to the breakpoint in the graph but once the fluid pressure on the master cylinder side exceeds that point, that on the wheel cylinder side increases as shown by the graph.





Its operation is as follows. Till the master cylinder side pressure (Pm) reaches breakpoint, the spring force (F) is higher than the force caused pressure (F = Pm S) and so Pm is transmitted to the wheel cylinder side as it is through the opening between the plunger and cup. As Pm rises higher than breakpoint, F. exceeds F and the plunger moves to the right as shown in the figure to close the above opening.
When Pm rises even higher, the plunger moves to allow the opening. Then the wheel cylinder side pressure (Pw) increases and the opening is closed.
Such operation cycle is repeated and Pm is transmitted to the wheel cylinder as Pw as shown by the above graph.





When the front brake line fails (when bleeding fluid from the brake line), the bypass line is opened to stop the proportioning valve so as to increase effect of the rear brake.





PURPOSE AND OPERATION OF DIFFERENTIAL SWITCH
For purpose and operation of this switch.