Check the emergency system for contamination. Remove the plug from the unused pneumatic pressure port on the brake assembly. Position a clean, white cloth adjacent to the opening, and slowly pull the emergency brake control handle. Allow airflow through the system for approximately 5 seconds. There should be no evidence of combustible contaminants on the cloth. If the system is contaminated, the emergency brake pneumatic lines from the brake control valve to the brake assembly must be flushed with a suitable solvent. Purge for a minimum of 15 minutes with heated nitrogen.

There are two general methods of bleeding brake systemsbleeding from top downward (top-down method) and bleeding from the bottom upward (bottom-up method). The method used generally depends on the type and design of the brake system to be bled. In some instances it may depend on the bleeding equipment available. A general description of each method is presented in the following paragraphs.

TOP-DOWN METHOD. In using the top-down method, the air is expelled from the system through one of the bleeder valves provided on the brake assembly. See figure 12-39. A bleeder hose is attached to the

Figure 12-39.-Bleeding brake system (top-down method).

bleeder valve, and the free end of the hose is placed in a container that has 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 a 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 or the bleeder hose pinched off; otherwise, more air will be drawn back into the system. Bleeding should continue until no more air bubbles come through the bleeder hose into the bleeder container.

BOTTOM-UP METHOD. In the bottom-up method, the air is expelled through the brake system reservoir or other specially provided location. Some aircraft have a bleeder valve located in the upper brake line. In this method of bleeding, pressure is supplied by a bleeder bomb. A bleeder bomb (fig. 12-40) is a portable tank in which hydraulic fluid is placed, and then put under pressure with compressed air. The bleeder bomb is equipped with an air valve, air gauge, and a connector hose. The connector hose, which attaches to the bleeder valve on the brake assembly, is provided with a shutoff valve. Normally, the hose is connected to the lowest bleed fitting on the brake assembly. With the brake bleed fitting opened, opening the bleeder bomb shutoff valve allows pressurized fluid to flow from the bleeder bomb through the brake system until all the trapped air is expelled. The brake bleeder valve is then secured, and the bleeder bomb hose is disconnected.

This method of bleeding should be performed strictly in accordance with specific instructions for the aircraft concerned. Although the bleeding of individual systems presents individual problems, the following precautions should be observed in all bleeding operations:

1. Ensure that the bleeding equipment is absolutely clean and filled with the proper type of hydraulic fluid.

2. Maintain an adequate supply of fluid during the entire operation. A low fluid supply will allow more air to be drawn into the system.

3. Continue bleeding until no more air bubbles are expelled from the system and a firm brake pedal is obtained.

4. Check the reservoir fluid level after the bleeding operation is completed. With brake pressure on, check the entire system for leaks.