Proportioning Valve: Operation

The piston is held pressed toward the left by the spring so that the valve is kept away from its seat.

Under this condition, fluid pressure "P3" to the rear wheel cylinders is equal to fluid pressure "P₂ " from the master cylinder.

Fig 1: Identifying Operation Before Split Point

Courtesy of SUBARU OF AMERICA, INC.

When pressure "P₂ " increases to the split point pressure, force "F₂ " is generated. (Piston's cross sectional area "A" has been selected so that the force is generated starting with the split point pressure.) The pressure pushes the piston rightward, overcoming spring force "F". As a result, the valve seat moves together with the piston rightward and comes in contact with the valve, blocking the passage toward the rear wheel cylinders.

Fig 2: Identifying Operation At Split Point Pressure

Courtesy of SUBARU OF AMERICA, INC.

Immediately before the fluid passage toward the rear wheel cylinders is closed, pressure "P₂ " is slightly higher than pressure "P₃ ". So the piston can move in the spring force acting direction and the fluid can flow to the wheel cylinders. However, as soon as pressure "P₂ " becomes equal to "P₃ ", the valve closes.

This cycle is repeated as long as the pedal is depressed further, but pressure increasing rate of the rear wheel cylinders is smaller than that of the front wheel cylinders.