Figure 12-71. Pneumatic brake system. Figure 12-72.  Pneumatic control valve. Air Compressors On some aircraft, permanently installed air compressors have been added to recharge air bottles whenever pressure  is used f or operating a unit. Several types of compressors are  used for this purpose. Some have two stages of compression,  while others have three, depending on the maximum desired  operating pressure. Relief valves are used in pneumatic systems to prevent  damage. They act as pressure limiting units and prevent  excessive pressures from bursting lines and blowing out seals.  Control valves are also a necessary part of a typical pneumatic  system.  Figure 12-72  illustrates how a valve is used to  control  emergency air brakes. The control valve consists of  three-port housing, two poppet valves, and a control lever  with two lobes. In  Figure 12-72A,  the control valve is shown in the off  position. A spring holds the left poppet closed so that  compressed air entering the pressure port cannot flow to the  brakes. In  Figure 12-72B,  the control valve has been placed  in the on position. One lobe of the lever holds the left poppet driven p ower pumps for building normal pressure. But  similarities do exist in some components.  open, and a spring closes the right poppet. Compressed air  now flows around the opened left poppet, through a drilled  passage, and into a chamber below the right poppet. Since  the right poppet is closed, the high-pressure air flows out of  the brake port and into the brake line to apply the brakes. To release the brakes, the control valve is returned to the  off position.  [Figure 12-72A]  The left poppet now closes,  stopping the flow of high-pressure air to the brakes. At the  same time, the right poppet is opened, allowing compressed  air in the brake line to exhaust through the vent port and into  the atmosphere. Relief Valves Control Valves 12-49 OFF ON HOLD Emergency pressure Emergency brake selector valve Emergency brake control knob Nonreturn valve Brake control valve Adjustable restrictor Parking handle Pressure reducing valve Air bottle Relief valve Shuttle valve Brake unit Brake pressure gauge Rapid exhaust valve Left pedal Right pedal Supply pressure To retraction and extension system Left wheels Right wheels Maxaret anti-skid unit A. Control valve “off” B. Control valve “on” Poppet valve Pressure port Drilled passage Drilled passage Control lever Vent port Spring Brake port From power source To emergency brakes