content_image/612193.jpg
applied. The tube expands outward, and the brake blocks make contact with the wheel drum causing friction that slows the wheel. As hydraulic pressure is increased, greater friction develops. Semi-elliptical springs located under the torque bars return the expander tube to a flat position around the flange when hydraulic pressure is removed. The clearance between the expander tube and the brake drum is adjustable by rotating an adjuster on some expander tube brakes. Consult the manufacturer’s maintenance manual for the correct clearance setting. Figure 13-91  gives an exploded view of an expander tube brake, detailing its components. 

Expander tube brakes work well but have some drawbacks. They tend to take a setback when cold. They also have a tendency to swell with temperature and leak. They may drag inside the drum if this occurs. Eventually, expander brakes were abandoned in favor of disc brake systems. 

## Brake Actuating Systems 

The various brake assemblies, described in the previous section, all use hydraulic power to operate. Different means of delivering the required hydraulic fluid pressure to brake assemblies are discussed in this section. There are three basic actuating systems: 

1. An independent system not part of the aircraft main hydraulic system; 

2. A booster system that uses the aircraft hydraulic system intermittently when needed; and 

3. A power brake system that only uses the aircraft main hydraulic system(s) as a source of pressure. 

Systems on different aircraft vary, but the general operation is similar to those described. 

## Independent Master Cylinders 

In general, small, light aircraft and aircraft without hydraulic systems use independent braking systems. An independent brake system is not connected in any way to the aircraft hydraulic system. Master cylinders are used to develop the necessary hydraulic pressure to operate the brakes. This is similar to the brake system of an automobile. 

In most brake actuating systems, the pilot pushes on the tops of the rudder pedals to apply the brakes. A master cylinder for each brake is mechanically connected to the corresponding rudder pedal (i.e., right main brake to the right rudder pedal, left main brake to the left rudder pedal). [Figure 13-92] When the pedal is depressed, a piston inside a sealed fluid- filled chamber in the master cylinder forces hydraulic fluid through a line to the piston(s) in the brake assembly. The brake piston(s) push the brake linings against the brake rotor to create the friction that slows the wheel rotation. Pressure 

<img src='content_image/612193.jpg'>

