TM 1-1500-204-23-2
Figure 4-148. Simple Master Cylinder
into the wheel brake assembly. Releasing the brake pedal retracts the master cylinder push rod, allowing the piston
spring to open the compensating valve and to hold it open until the pedal is depressed again.
(b)
Compound master cylinder. Some aircraft require wheel brake systems that operate with
more fluid flow and higher fluid pressure than is within the capability of a simple master cylinder. In some types of
aircraft, a compound master cylinder is used for this purpose. This type of master cylinder can deliver a large quantity of
fluid at low pressure by means of relatively short pedal travel. It can also develop high operating pressure by means of
moderate pedal pressure This dual capability is achieved by means of two pistons: a large one for producing a large fluid
flow with short travel; and a small one for developing high pressure from a lightly applied force. Figure 4-149 shows a
compound master cylinder in a released position, which means that the compensating valves of both pistons are open to
the reservoir.
1
Brakes applied. As the brake pedal is depressed, both compensating valves
close and the pistons are forced ahead in their cylinder bores. The fluid displaced jointly by the two pistons flows Into the
wheel brake assembly, where it moves the linings into contact with the disk. A slight additional movement of the brake
pedal and pistons raises the pressure in the system by a few pounds. This slight rise in pressure moves a spring-loaded
spool in the transfer valve within the master cylinder. The new position of the spool blocks the flow of fluid from the large
cylinder to the wheel brake assembly and opens a passage between the large cylinder bore and the reservoir, thus
allowing any fluid displayed by the large piston to flow without resistance into the reservoir. The small cylinder bore
remains open to the passage within the master cylinder through which fluid is delivered to the wheel brake assembly. As
force applied to the brake pedal is increased so as to push the linings into hard contact with the brake disc, the two
pistons move slightly farther into their bores. Since any fluid displaced out of its bore by the large piston is flowing to the
reservoir without resistance, practically all the force exerted on the brake pedal is applied to the small piston.
2
Brakes released. Upon release of the brake pedal, the spool of the transfer valve
returns to is spring-loaded position, the pistons return to their released position, the compensating valves open, and the
fluid that was forced into the wheel brake assembly when the brake was applied returns to the master cylinder.
(c)
Power-boost master cylinder. Some aircraft land at speeds too great to permit control of
the aircraft by a brake system powered entirely by an operator's foot and leg muscles. The brake systems of such
aircraft require power produced by a power-driven pump In some aircraft, the pump-produced power does the job alone
and is merely controlled by the operator In other aircraft, the pump-produced power is used as a supplement (or boost) to
the power supplied by the muscles of the operator; the power-boost master cylinder is used in these brake systems In the
event of
4-144
