return check valve allows fluid to flow back to the master
cylinder when the brakes are released and system pressure
becomes higher than master cylinder pressure, Figure 9-11.
However, the return check valve contains a spring which
opposes returning fluid pressure. This spring is designed to
close the valve when the system pressure goes below a
certain point.
When the brakes are applied, the apply check valve
unseats and fluid flows to the wheel units. When the
brakes are released, the apply check valve closes, and
fluid returns to the master cylinder through the return
check valve. When the system pressure becomes lower
than spring pressure, the spring closes the valve. This
traps a small amount of pressure in the brake hydraulic
Pressure Differential Valve
The pressure differential valve is a warning device
used in all split brake systems. Remember that a split brak-
ing system contains two separate brake hydraulic systems
or sides. The pressure differential valve is installed so that
each side of the hydraulic system presses on one side of
the valve, Figure 9-12. Attached to the valve is an electrical
switch. This switch receives electrical power whenever the
ignition is on. The switch’s electrical circuit goes through a
dashboard-mounted brake warning light. To complete the
circuit, the switch must be grounded by the pressure differ-
ential valve.
When the hydraulic system is operating normally,
pressure on both sides of the valve will be equal. With the
pressure on both sides equal, the valve is centered and the
switch is open. When one side of the hydraulic system fails,
pressing on the brake pedal will result in normal pressure
on one side, and much lower pressure on the other side of
the system. The difference in pressure pushes the valve to
move toward the side with less pressure, Figure 9-13.
When the valve moves, it moves the switch plunger,
causing the switch contacts to close. This grounds the
switch, allowing current flow to turn on the dashboard
warning light.
Combination Valve
For manufacturing ease and quicker service, two or
more hydraulic valves are often combined into one assem-
bly. This multiple valve assembly is called a combination
valve. A combination valve may contain up to three differ-
ent hydraulic valves. Common two-unit combination
valves usually contain a pressure differential valve and
either a metering valve or proportioning valve, Figure 9-14.
A few two-unit combination valves contain the metering
and proportioning valves. Three-unit combination valves
are composed of a metering valve, proportioning valve,
and pressure differential valve, Figure 9-15.
156 Auto Brakes
Figure 9-11. The brakes have been released and the return
check valve is being forced open. Fluid is now flowing back into
the master cylinder. (ATE)
Return check
Master cylinder
Figure 9-12. A cross-sectional view of a pressure differential
valve with equal pressure on both sides of the piston. (Niehoff)
To front wheels
via metering valve
To brake warning light
Fr rom master
To rear
Figure 9-13. Cutaway of a pressure differential valve and warn-
ing switch. Note that there has been a failure in the front brake
system. The master cylinder pressure has moved the piston to
the left, making switch contact and turning on the dash warning
light. (Chevrolet)
From master cylinder
To o front
wheels w
ar To r ea
els wh ee
Piston Switch
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