maintained at all times to prevent brake failure or the introduction of air into the system. Air in the system is indicated by
a spongy action of the brake pedals. If air is present in the system, it must be removed by bleeding.
Bleeding. There are two general methods of bleeding brake systems: pressure bleeding, and
gravity bleeding. The method used usually depends on the type and design of the brake system, but may be dictated by
availability of required equipment. Specific instructions are found in the aircraft technical manual.
Pressure bleeding. In the pressure method, shown in figure 4-160, air is expelled
through the brake system reservoir or a bleeder valve located somewhere within the system. Pressure is supplied by a
pressurized hydraulic fluid dispenser. Pressurized fluid flows completely through the system, unit all air is expelled.
Gravity bleeding. In the gravity method, the air is expelled from the system through one
of the bleeder valves on the brake assembly. As shown in figure 4-161, a bleeder hose is attached to the bleeder valve
and the free end of the hose is placed in a container holding enough hydraulic fluid to cover the end of the hose. The air-
laden fluid is then forced from the system by applying the brakes. If the brake system is part of the main hydraulic
system, a portable hydraulic test stand may be used to supply the pressure. If the system is an independent master
cylinder system the master cylinder will supply the necessary pressure. In either case, each time the brake pedal is
released, the bleeder valve must be closed; otherwise air will be drawn back into the system. Bleeding should continue
until no more air bubbles come through the bleeder hose into the container.
Troubleshooting. Table 4-41 lists common troubles found in wheel brake systems, and table 4-
42 describes those found in helicopter rotor brake systems used on Army aircraft.
Figure 4-159. Rotor Blade Brake System