Master cylinder Clear hose Brake reservoir Figure 13-119.  Arrangement for top down or gravity bleeding of  aircraft brakes.  13-70 fluid to enter the brake system. The fluid expels the fluid  contaminated with air out of the bleed hose into the container.  When air is no longer visible in the hose, close the bleed  valve and restore the hydraulic system to normal operation  configuration.  Power brake systems on different aircraft contain many  variations and a wide array of components that may affect  the proper bleeding technique to be followed. Consult the  manufacturer’s maintenance information for the correct  Many aircraft with power brake systems accept the  connection of an auxiliary hydraulic mule that can be used  to establish pressure in the system for bleeding. Regardless,  the aircraft system must be pressurized to bleed power brake  systems. Attach a clear hose to the brake bleed port fitting on  the brake assembly and immerse the other end of the hose in  a container of clean hydraulic fluid. With the bleeder valve  open, carefully apply the brake to allow aircraft hydraulic  Top down brake bleeding is used in power brake systems.  Power brakes are supplied with fluid from the aircraft  hydraulic system. The hydraulic system should operate  without air in the fluid as should the brake system. Therefore,  bottom up pressure bleeding is not an option for power  brakes. The trapped air in the brake system would be forced  into the main hydraulic system, which is not acceptable. Whenever bleeding the brakes, ensure that reservoirs and  bleed tanks remain full during the process. Use only clean,  specified fluid. Always check the brakes for proper operation,  any leaks when bleeding is complete, and assure that the fluid  quantity level is correct. Bleeding Power Brake Systems Brakes with master cylinders may also be gravity bled  from the top down. This is a process similar to that used  on automobiles.  [Figure 13-119]  Additional fluid is  supplied to the aircraft brake reservoir so that the quantity  does not exhaust while bleeding, which would cause the  reintroduction of more air into the system. A clear hose is  connected to the bleed port on the brake assembly. The other  end is submersed in clean fluid in a container large enough to  capture fluid expelled during the bleeding process. Depress  the brake pedal and open the brake assembly bleed port. The  piston in the master cylinder travels all the way to the end of  the cylinder forcing air fluid mixture out of the bleed hose  and into the container. With the pedal still depressed, close  the bleed port. Pump the brake pedal to introduce more fluid  from the reservoir ahead of the piston in the master cylinder.  Hold the pedal down and open the bleed port on the brake  assembly. More fluid and air is expelled through the hose into  the container. Repeat this process until the fluid exiting the  brake through the hose no longer contains any air. Tighten  the bleed port fitting and ensure the reservoir is filled to the  proper level.  the bleed port and pressure tank shutoff are closed, and the  pressure tank hose is removed. The hose at the reservoir is also  removed. Fluid quantity may need to be adjusted to assure the  reservoir is not over filled. Note that it is absolutely necessary  that the proper fluid be used to service any brake system  including when bleeding air from the brake lines. 