pressure or return port of the brake line. Two springs are
provided. The large spring, referred to in the illustration
as the plunger spring, provides feel to the brake pedal.
The small spring returns the spool to the OFF position.
When the plunger is depressed, the large spring
moves the spool, which closes off the return port and
opens the pressure port to the brake line. When the
pressure enters the valve, fluid flows to the opposite end
of the spool through a hole. The pressure pushes the
spool back far enough toward the large spring to close
the pressure port, but not open the return port. The valve
is then in the static condition. This movement partially
compresses the large spring, giving feel to the brake
pedal. When the brake pedal is released, the small spring
moves the spool back, opening the return port. This
action allows fluid pressure in the brake line to flow out
through the return port.
Maintenance of the sliding spool brake control
valve is limited to checking the action of the plunger.
This is done by manually depressing the plunger until it
bottoms, and then releasing it suddenly. If the plunger
remains depressed (does not snap out), the valve is
binding at the spool and sleeve. If binding occurs, the
valve should be replaced. Disassembly of the valve is
not permitted at the organizational level of maintenance,
but may be performed by an intermediate or higher level
Brake Debooster Cylinder
In some power brake control valve systems,
debooster cylinders are used in conjunction with the
power brake control valves. These units are generally
used on aircraft equipped with a high-pressure hydraulic
system and low-pressure brakes. The purpose of the
brake debooster cylinder is to reduce the pressure to the
brake and increase the volume of fluid flow. Figure
12-29 shows a typical debooster cylinder installation.
The unit is being mounted on the landing gear shock
strut in the line between the control valve and the brake.
The schematic diagram in the illustration shows the
internal parts of the cylinder.
When the brake is applied, fluid under pressure
enters the inlet port to act on the small end of the piston.
The ball check prevents the fluid from passing through
the shaft. Force is transmitted through the small end of
the piston to the large end of the piston. As the piston
moves downward in the housing, a new flow of fluid is
created from the large end of the housing through the
outlet port to the brake. Because the force from the small
piston head is distributed over the greater area of the
Main brake pressure line
Upper support clamp
Piston return spring
Lower support clamp
Brake line (to pressure
23. Spring retainer
24. Valve spring
26. Ball pedestal
28. Lower cud cap
Brake tine (debooster to
Brake shuttle valve
29. Outlet port
Figure 12-29.-Brake debooster cylinder.
large piston head, pressure at the outlet poet is reduced.
At the same time, a greater volume of fluid is displaced
by the large piston head than that used to move the small
Normally, the brake will be fully applied before the
piston has reached the lower end of its travel. However,
if the piston fails to meet sufficient resistance to stop it
(due to a loss of fluid from the brake unit or connecting
lines), the piston will continue to move downward until
the riser unseats the ball check valve in the hollow shaft.
With the ball check valve unseated, fluid from the power
control valve will pass through the piston shaft to