FED-STD-791D
6.5.1.3 Leakage (or evaporation). The leakage (or evaporation) loss of the hydraulic
brake fluid at 24,000-stroke intervals shall be determined by the mL of the fluid required to
maintain the constant fluid level in the master cylinder reservoir. The level of the fluid should be
checked daily.
6.6 Observations and measurements at end of test.
6.6.1 Leakage. The equipment shall be allowed to come to room temperature within 16
h following completion of the required number of strokes. The brake wheel cylinder shall be
examined for evidence of excessive leakage. The assembly shall then be stroked an additional
100 strokes at 3.4 gage MPa (500 psig), and the cylinders again examined for evidence of
excessive leakage.
6.6.2 Disassembly of cylinders. The wheel cylinders and the master cylinder shall be
removed from the simulated system. The inlet ports of the cylinders shall be plugged or capped
immediately upon removal to retain the fluid remaining therein. Each cylinder shall then be
carefully disassembled. The fluid remaining in the cylinder shall be drained into a small
screwcap jar, and saved for examination for evidence of sludging, jelling, sedimentation, and
grittiness.
6.6.3 Inspection. A visual inspection of the disassembled cylinder shall be made, and a
record made of the type and quantity of dirt; gum deposition; rusty, scored, and galled areas on
the surface of the pistons and on the cylinder walls; chipping, scoring, and general condition of
the rubber cups. All parts shall then be cleaned with 95% ethyl alcohol, and dried in a stream of
air. The metal parts shall then be re-examined for evidence of corrosive action, such as pitting,
etching, and severe stain.
6.6.4 Measurements.
6.6.4.1 Cylinders. The internal diameter of each wheel cylinder and of the master
cylinder shall be measured as specified in 6.2.1.
6.6.4.2 Pistons. The external diameter of the master cylinder piston and of each wheel
cylinder piston shall be measured as specified in 6.2.2.
6.6.4.3 Rubber cups. Measurements of the base and lip diameters of the rubber cups
shall be made as specified in 6.2.3.1. Durometer hardness of the cups shall be determined as
specified in 6.2.3.2. The diameter measurements and the hardness determinations shall be made
within 1 h following removal from the cylinder assembly.
7. CALCULATIONS
7.1 Change in cylinder and piston diameter:
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