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. 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. Systems on different aircraft vary, but the general operation  is similar to those described.  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:  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  Independent Master Cylinders  Brake Actuating Systems 3. A power brake system that only uses the aircraft main  hydraulic system(s) as a source of pressure.  2. A booster system that uses the aircraft hydraulic  system intermittently when needed; and  1. An independent system not part of the aircraft main  hydraulic system;  13-52 Figure 13-91. An exploded view of an expander tube brake. Torque bar bolt Brake frame bolt Torque flange Torque bar Frame Frame Expander tube Brake block Return spring Tube shield Nozzle Return spring shield Adjuster Adjuster bolt