13-66 Control Unit Brake Inspection and Service In-Flight Test  Control Valve Ground Test Wheel Speed Sensor Anti-Skid System Tests Auto Brakes Anti-Skid System Maintenance Anti-skid components require little maintenance.  Troubleshooting anti-skid system faults is either performed  via test circuitry or can be accomplished through isolation  of the fault to one of the three main operating components of  the system. Anti-skid components are normally not repaired  in the field. They are sent to the manufacturer or a certified  repair station when work is required. Reports of anti-skid  system malfunction are sometimes malfunctions of the brake  system or brake assemblies. Ensure brake assemblies are bled  and functioning normally without leaks before attempting to  isolate problems in the anti-skid system. Aircraft equipped with auto brakes typically bypass the brake  control valves or brake metering valves and use a separate  auto brake control valve to provide this function. In addition  to the redundancy provided, auto brakes rely on the anti-skid  system to adjust pressure to the brakes if required due to an  impending skid.  Figure 13-112  shows a simplified diagram  of the Boeing 757 brake system with the auto brake valve in  relation to the main metering valve and anti-skid valves in  this eight-main wheel system. It is important to know the status of the anti-skid system prior  to attempting to use it during a landing or aborted takeoff.  Ground tests and in-flight tests are used. Built-in test circuits  and control features allow testing of the system components  and provide warnings should a particular component or  part of the system become inoperative. An inoperative  anti-skid system can be shut off without affecting normal  brake operation. Wheel speed sensors must be securely and correctly mounted  in the axle. The means of keeping contamination out of the  sensor, such as sealant or a hub cap, should be in place and in  good condition. The wiring to the sensor is subject to harsh  conditions and should be inspected for integrity and security.  It should be repaired or replaced if damaged in accordance  with the manufacturer’s instructions. Accessing the wheel  speed sensor and spinning it by hand or other recommended  device to ensure brakes apply and release via the anti-skid  system is common practice.  Ground tests vary slightly from aircraft to aircraft. Consult  the manufacturer’s maintenance manual for test procedures  specific to the aircraft in question.  Much of the anti-skid system testing originates from testing  circuits in the anti-skid control unit. Built-in test circuits  continuously monitor the anti-skid system and provide  warning if a failure occurs. An operational test can be  performed before flight. The anti-skid control switch and/ or test switch is used in conjunction with system indicator  light(s) to determine system integrity. A test is first done  with the aircraft at rest and then in an electrically simulated  anti-skid braking condition. Some anti-skid control units  contain system and component testing switches and lights  for use by the technician. This accomplishes the same  operational verification but allows an additional degree of  troubleshooting. Test sets are available for anti-skid systems  that produce electric signals that simulate speed outputs of  the wheel transducer, deceleration rates, and flight/ground  parameters.  Anti-skid control valve and hydraulic system filters should  be cleaned or replaced at the prescribed intervals. Follow  all manufacturer’s instructions when performing this  maintenance. Wiring to the valve must be secure, and there  should be no fluid leaks. Control units should be securely mounted. Test switches  and indicators, if any, should be in place and functioning. It  is essential that wiring to the control unit is secure. A wide  variety of control units are in use. Follow the manufacturer’s  instructions at all times when inspecting or attempting to  perform maintenance on these units.  Brake inspection and service is important to keep these  critical aircraft components fully functional at all times.  There are many different brake systems on aircraft. Brake  system maintenance is performed both while the brakes are  installed on the aircraft and when the brakes are removed.  The manufacturer’s instructions must always be followed to  ensure proper maintenance.  In-flight testing of the anti-skid system is desirable and  part of the pre-landing checklist so that the pilot is aware of  system capability before landing. As with ground testing, a  combination of switch positions and indicator lights are used  according to information in the aircraft operations manual. 