bleeding procedure for each aircraft. Be sure to bleed  auxiliary and emergency brake systems when bleeding the  normal brake system to ensure proper operation when needed.  As mentioned, it is imperative that the correct hydraulic fluid  is used in each brake system. Seals in the brake system are  designed for a particular hydraulic fluid. Deterioration and  failure occurs when they are exposed to other fluids. Mineral- based fluid, such as MIL-H-5606 (red oil), should never be  mixed with phosphate-ester based synthetic hydraulic fluid,  such as Skydrol®. Contaminated brake/hydraulic systems  must have all of the fluid evacuated and all seals replaced  before the aircraft is released for flight. Fluid quantity is also important. The technician is responsible  for determining the method used to ascertain when the  brake and hydraulic systems are fully serviced and for  the maintenance of the fluid at this level. Consult the  manufacturer’s specifications for this information. Aircraft brake systems should maintain all fluid inside lines  and components and should not leak. Any evidence of a leak  must be investigated for its cause. It is possible that the leak is  a precursor to more significant damage that can be repaired,  thus avoiding an incident or accident.  [Figure 13-120] Many leaks are found at brake system fittings. While  this type of leak may be fixed by tightening an obviously  loose connection, the technician is cautioned against over- tightening fittings. Removal of hydraulic pressure from the  brake system followed by disconnection and inspection of  the connectors is recommended. Over-tightening of fitting  can cause damage and make the leak worse. MS flareless  All bolts and threaded connections are inspected. They should  be in good condition without signs of wear. Self-locking  nuts should still retain their locking feature. The hardware  should be what is specified in the brake manufacturer’s  parts manual. Many aircraft brake bolts, for example, are  not standard hardware and may be of closer tolerance or  made of a different material. The demands of the high stress  environment in which the brakes perform may cause brake  failure if improper substitute hardware is used. Be sure to  check the condition of all threads and O-ring seating areas  machined into the housing. The fittings threaded into the  housing must also be checked for condition. Brake discs must be inspected for condition. Both rotating  and stationary discs in a multiple disc brake can wear. Uneven  wear can be an indication that the automatic adjusters may  not be pulling the pressure plate back far enough to relieve  all pressure on the disc stack.  Stationary discs are inspected for cracks. Cracks usually  extend from the relief slots, if so equipped. On multiple disc  brakes, the slots that key the disc to the torque tube must  also be inspected for wear and widening. The discs should  engage the torque tube without binding. The maximum width  of the slots is given in the maintenance manual. Cracks or  Certain servicing and maintenance of an aircraft brake  assembly is performed while it has been removed from the  aircraft. A close inspection of the assembly and its many parts  should be performed at this time. Some of the inspection  items on a typical assembly follow. The stress experience by the landing gear and brake system  requires that all bolts are properly torqued. Bolts used to  attach the brakes to the strut typically have the required  torque specified in the manufacturer’s maintenance manual.  Check for torque specifications that may exist for any landing  gear and brake bolts, and ensure they are properly tightened.  Whenever applying torque to a bolt on an aircraft, use of a  calibrated torque wrench is required. fitting are particularly sensitive to over-tightening. Replace  all fittings suspected of damage. Once any leak is repaired, the  brake system must be re-pressurized and tested for function  as well as to ensure the leak no longer exists. Occasionally,  a brake housing may seep fluid through the housing body.  Consult the manufacturer’s maintenance manual for limits  and remove any brake assembly that seeps excessively. Proper Bolt Torque  Fluid Quantity and Type Off Aircraft Brake Servicing and Maintenance Bolt and Threaded Connections  Inspection for Leaks Discs 13-71 Figure 13-120.  The cause of all aircraft brake leaks must be  investigated, repaired, and tested before releasing the aircraft  for flight.