When the vehicle is lightly loaded, the rear will rise.
This increases the distance between the axle and body,
causing the link to extend and lower the pressure sent to
the rear wheels. A delay mechanism is built into the pro-
portioning valve to keep quick axle movements (such as
those caused by bumps) from affecting the valve.
Residual Pressure Valve
On any vehicle with drum brakes, a small amount of
pressure must be maintained in the system to keep the
wheel cylinder cup lips from collapsing, usually about
7 psi (47 kPa). All vehicles with drum brakes have a
residual pressure valve to maintain this pressure.
Figure 9-8 shows a residual pressure valve installed in the
end of the master cylinder bore.
Most modern vehicles with drum brakes have a resid-
ual pressure valve installed in the outlet to the rear wheels,
Figure 9-9. Older vehicles have residual pressure valves
installed in each outlet. While this leaves a slight residual
pressure on the entire brake system, it is not enough to
cause brake drag or accidental brake apply. Some manu-
facturers feel the small amount of residual pressure makes
initial brake application slightly quicker.
The residual pressure valve consists of two check
valves. The apply check valve allows fluid flow to the rest
of the brake system whenever master cylinder pressure is
greater than system residual pressure, Figure 9-10. The
Chapter 9 Hydraulic Valves, Switches, Lines, and Hoses 155
Figure 9-8. A master cylinder cutaway showing the residual
pressure check valve assembly and location. (Raybestos)
Fluid Reservoir
Piston return Pisto
sppring
Residual pressure
check valve
Trapped brake
fluid under
pressure
Front
outlet
port
Figure 9 Figure 9 -9. 9. A master cylinder with a residual pressure check valve for use with rear drum brakes. (Wagner) A master cylinder with a residual pressure check valve for use with rear drum brakes. (Wagner)
Reservoir Reservoir
cover cover
*Manual Brake Master Cylinders Only
Spring
Cup retainer
Primary cup
Diaphragm
Tube seat
insert
Residual pressure
check valve
Check valve
spring
Piston stop
bolt
Primary piston
*Pushrod retainer a
*Pushrod retainer
(variation)
*Pushrod
*Boot
*Washer a
Lock ring
Secondary
piston
Secondary
cups
Cup protector
Figure 9-10. Cutaway of one type of residual pressure valve
showing the apply valve allowing fluid pressure to travel from
the master cylinder to the entire system. (ATE)
Apply check
valve
Master cylinder body