TM 1-1500-204-23-2
Figure 4-149. Compound Master Cylinder
loss of pump-produced power to the master cylinder, some brake application by muscle power alone is possible.
However, considerably more force than normal has to be applied to the brake pedal to achieve the necessary braking
action. In heavy aircraft the most vigorous manual applicator is still completely inadequate and an emergency source of
boost pressure, such as an accumulator or a compressed-air bottle, has to be provided.
1
Design. Figure 4-150 shows a typical power-boost master cylinder. Note that
two interconnected chambers make up the cylinder body The upper chamber houses a spring-loaded sleeve and slide
and the pressure and return ports for the main system lines. The lower chamber contains a spring-loaded piston and
poppet and the port to the brake line. This chamber also serves as a reservoir for the fluid that energizes the brake when
pressure is applied. An actuating lever and two slide actuating springs are mounted on top of the cylinder. One end of
the actuating lever is secured to the aircraft, the other end, to the cylinder housing. It should be noted that the master
cylinder described here has a piston rod that is pulled outward of the housing as the brake is applied. However, there is
another design in which applying the brake pushes the piston rod inward.
2
Fluid pressurization. Applying pressure to the brake pedal creates a pull on the
piston rod that results in downward movement of the piston. When sufficient movement of the piston occurs to release
the poppet extension from its contact with the housing, the poppet seats, thereby closing the compensating valve. Fluid
trapped between the underside of the piston and the wheel brake assembly then becomes pressurized. This fluid
pressure, acting with force exerted by the piston retracting spring, creates a downward pull on the master cylinder
housing. When the pull becomes sufficiently great, it causes the pivoted actuating lever to move from its previously
angled position. The new position of the actuating lever forces the attached slide actuating springs to retract the slide,
thereby causing the slide to block the return port and to open the pressure port. This allows pressure from the system
pump to pass through the pressure port and the slide and to enter the chamber above the piston. This pressure exerts a
downward push on the piston, which aids the muscle (operator) produced pull on the piston in developing brake
application pressure within the lower chamber and the connected wheel brake assembly. Whatever pressure is present
in the chamber above the piston is also present within the slide and tends to force the slide upward against the resistance
of the slide actuating springs. When the pressure-produced force
4-145
