TM 1-1500-204-23-2
Figure 4-150. Power-Boost Master Cylinder (Brake Applied)
acting on the slide becomes slightly greater than the force exerted by the slide actuating springs, the slide moves upward
to the extent that it blocks both the pressure and return ports.
3
Pressure maintenance. Since no pressure can then be relieved or increased
within the chamber above the piston, brake application remains constant as long as the operator does not change the
force applied to the brake pedal. The degree of brake application is under complete control of the operator and is
determined by the amount of pressure he applies to the brake pedal.
4
Brake release. Release of pressure on the brake allows the piston spring to
retract the piston, which results in reopening the compensating valve (see figure 4-151). This allows the actuating lever
to return to its angled position, thereby permitting the slide to return to the position, thereby permitting the slide to return
to the position in which it blocks the pressure port and opens the return port. Fluid that was forced Into the wheel brake
assembly during brake application can then return to the chamber on the underside of the piston and thus release the
brake.
(2)
Power control valves. In aircraft that land at high speed, power brake control valves are used
instead of master cylinders to provide the wheel brake assemblies with fluid pressure. The main difference between a
power brake control valve and a master cylinder lies in the pressure source. A power brake control valve produces no
pressure. It is pressure to a wheel brake assembly. Since the power brake control valve does not produce pressure, a
brake system using this type of valve must be supplied with an emergency source of input pressure, such as an air bottle,
for use in the event that the regular pressure source fails.
(a)
Brake application. When the brake pedal is depressed, the plunger moves toward the
left as shown in figure 4-152. This movement is transmitted to the spool by the spool pressure spring causing the spool
to move toward the left if the brake pedal is depressed only a minimum amount, the spool's leftward movement is just
enough to barely unblock the pressure port. This movement of the spool interconnects the pressure and brake lines and
isolates the brake line from the return line. As a result fluid flows out of the brake port into the brake assembly, thus
raising the pressure in the wheel brake assembly and causing the brake to be applied. The rise in pressure in the wheel
brake assembly takes place gradually because of the very small opening between the spool and the pressure
4-146
