Subaru XT 1988 Service Manual page 953

4-4
BRAKES
OPERATION
OPERATION DURING NORMAL CONDITIONS
1. Prior to valve operation (Before split point)
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.
2. After valve operation
(~fter
split point)
1) When the master cylinder fluid pressure rises, the piston in
the primary circuit is moved rightward 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 8), and the fluid pressure to the
rear wheel cylinder is thus controlled. (The pressure at this
moment is the split point pressure.)
If the fiuid pressure in chamber A rises further, the piston is
moved leftward, off the seal (1
L
and this causes the fluid
pressure in chamber B to rise. The piston is then moved right·
ward, 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.
OPERATION IN CASE OF CIRCUIT FAILURE
1. Failure of primary circuit
If the primary circuit fails, the fail·safe piston and balance
piston are moved rightward by the fluid pressure in the master
cylinder in the secondary circuit until the piston contacts the
plug. In th is case, the balance piston remains off the seal (2),
and no split point is created in the graph. That is, the flu id
pressure in the secondary side rear wheel cylinder is equal to
the fluid pressure in the master cylinder.
To rear wheel cylinder
To rear wheel cylinder
o
o
(inoperative)
Seal (2)
Fig. 16
o
From master cylinder
(Secondary)
Plug
j,
OFrom master cylinder
(Primary)
(inoperative) L4.915
2) When the fluid pressure in chamber B is controlled in the
2. Failure of secondary circuit
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 0 is controlled. Since sectional areas A 1
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 0 and chamber B by repeating open·close
operation with the seal (2).
To rear wheel cylinder
o
Balance
A2
To rear wheel cylinder
{ ) Seal (1)
{ ) Chamber (B)
0
From master cylinder (Secondary)
Fig. 15
From master cylinder
(Primary)
L4·914
6
If the secondary circuit fails, the balance piston is moved
leftward by the fluid pressure in chamber B until the end of
the piston contacts the stopper. Since sectional area A 1 is
greater than A2, the piston remains unmoved even after the
master cylinder fluid pressure has reached the split point, and
the piston is kept off the seal (1). Hence, no split point is
created in the graph, and the rear wheel cylinder fluid pressure
of the primary circuit is kept equal to the master cylinder
fluid pressure.
To rear wheel cylinder
linoperativeb Chamber B
piston
From master
cylinder
(Secondary)
(i noperative)
Fig. 17
To rear wheel cylinder
Stopper
From master cylinder
(Primary)
L4·916