Outback [Legacy] AWD F4-2.5L DOHC (1996)
Brake Proportioning/Combination Valve: Description and Operation
Without ABS
PROPORTIONING VALVE
The proportioning valve for dual piping Systems is adopted for controlling the braking force.
Operation During Normal Conditions
When the fluid pressure in the master cylinder is low (the fluid pressure before the split point), the piston is pressed by the spring load and the
valve remains inoperative. As a result, the fluid pressure in the master cylinder is held equal to the fluid pressure in the rear wheel cylinder.
1) When the master cylinder fluid pressure rises, the piston in the primary circuit is moved right- ward against the spring load, and brought into
contact with the seal (1) (as shown in the figure). The master cylinder fluid pressure chamber (chamber A) is therefore cut off from the rear
wheel cylinder fluid pressure chamber (chamber B), and the fluid pressure to the rear wheel cylinder is thus controlled. (The pressure at this
moment is the split point pressure.)
If the fluid pressure in chamber A rises further, the piston is moved leftward, off the seal (1), and this causes the fluid pressure in chamber B to
rise. The piston is then moved rightward, and brought into contact with the seal (1) again. After this, the piston repeats this contact with the
seal (1) in this way, thereby controlling the fluid pressure in the rear wheel cylinder.
2) When the fluid pressure in chamber B is controlled in the secondary circuit, the balance piston is moved rightward by the fluid pressure
difference between chamber B and chamber C, and brought into contact with the seal (2), and the fluid pressure in chamber D is controlled.
Since sectional areas A1 and A2 are equal, the balance piston is pushed by equal forces from the right and left. If the fluid pressure rises in
chamber B, the balance piston performs control to equalize the fluid pressure in chamber D and chamber B by repeating open-close operation
with the seal (2).
