Subaru XT 1988 Service Manual page 950

BRAKES
4-4
Primary hydraulic chamber (Chamber P)
Fig.
3
Large diameter side
hydraulic chamber
(Chamber F)
L4-OO9
OPERATION OF FAST-FILL MECHANISM
When compared to a conventional master cylinder, the one
with the fast-fill mechanism has the following two unique
structural features:
1) It has a large diameter chamber (chamber F) on the inlet
side of the master cylinder, and small diameter chambers
(chambers P and S) on the inner side. The primary piston is
of a stepped construction with those two diameters.
2) A check valve, which is called the fast-fill valve (FFV), is
installed between chambers P & F and the reservoir.
This FFV controls the flow of brake fluid from chamber F to
reservoir, and vice versa, by ball valve and supply valve,
respectively.
Notch
The ball valve is opened when fluid pressure in chamber F
exceeds the preset pressure level. The notch in the supply
valve keeps the valve always in a partially opened state, so that
no fluid pressure will be left in chambers P and F when the
system is not operating.
Operation of fast-fill valve
1) Low fluid pressure period
During the low fluid pressure period which begins immediately
after starting the piston and ends when the ball valve in the
FFV releases, the fluid pressure generated in chamber F is
higher than the one in chamber P; the fluid pressure difference
causes fluid to flow from chamber F to chamber P. The
amount of fluid flow shortens the brake pedal stroke, i.e.
brake fluid is pushed out of the master cylinder through the
sectional area of chamber F.
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Piston movement
L4-011
Fig.
5
2) High fluid pressure period
During the high fluid pressure period after releasing fluid from
the ball valve, the pressure in chamber P is higher than the
pressure in chamber F, and flow of fluid is made stationary by
primary cup's sealing the passage. As the fluid pressure in
chamber F drops thereafter, the operating pressure is generated
by the pedal depressing force and the sectional area of
chamber P.
An orifice is provided in the passage between chamber F and
the reservoir for smooth pedal feel.
Supply valve--
¢:l
Piston movement
L4-010 L4-012
L-____________________________________________
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3