FED-STD-791D
6.2.3.1 Base and lip diameters of the cups. The internal base and lip diameters of each
rubber cup shall be determined with a micrometer, to the nearest 0.0025 cm (0.001 in). The cups
shall be placed on a flat surface for 24 h before measuring. In measuring the diameters of the
cups, care should be taken that the micrometer does not extend more than 0.08 cm (1/32 in)
beyond the edges of the cup. If difficulty is encountered in measuring the diameter of the master
cylinder secondary cup, the cup may be placed on a mandrel having a slight taper. Two readings
at right angles to each other, shall be taken on the base and lip of each cup, and their average
considered as the base and lip diameters of the cup. If the two readings on the base or the two
readings on the lip differ by more than 0.0025 cm (0.001 in) respectively, the cup in question
shall be discarded.
6.2.3.2 Hardness of rubber. The durometer hardness of each cup shall be measured at
four different points near the center of each cup. The average of the four readings shall be
considered as the hardness of the rubber cup. For the sake of uniformity the durometer readings
should be made as follows: The proper rubber anvil is inserted in the fixture, and the rubber cup
placed on the anvil. The back edge of the durometer is placed on the shelf of the fixture, with the
bottom surface of the durometer kept parallel to the surface of the base of the cup. The
durometer is grasped between the thumb and the third and fourth fingers of one hand, with the
index finger placed on the top of the depressor. The indentor point is lowered gently until it
rests on the surface of the cup. With a minimum amount of rocking, pressure is then applied so
that the pressure plate makes firm contact with the surface of the cup. The durometer reading is
taken as soon as the dial needle comes to rest. If the needle does not come to rest, then the
reading is taken at that moment when a definite change in the speed of the needle is observed,
with the reading estimated to the nearest whole point of the dial scale.
6.2.3.2.1 Wheel cylinder cups. The average durometer hardness of the wheel cylinder
cups shall be 60 " 3 points. Nonconforming cups shall be discarded.
6.2.3.2.2 Master cylinder cups. The average durometer hardness of the master cylinder
cups shall be 63 " 3 points. Nonconforming cups shall be discarded.
6.3 Assembling, adjusting, and checking the apparatus.
6.3.1 Assembly of brake parts. After the required measurements have been taken and
recorded, the wheel cylinders and the master cylinder shall be assembled for test. All parts with
the exception of the cylinder housings and the rubber boots, shall be dipped in the hydraulic fluid
under test when they are assembled. The assembled cylinders shall be checked for ease of
operation, and then installed in their respective places in the simulated brake systems. Helper
springs shall be installed in holes beneath the shoe flanges, pushed up and hooked over the
flanges (see Figure 1).
6.3.2 Shoe clearance adjustment. The brake shall be adjusted to have a clearance of
0.102 " 0.0075 cm (0.040 " 0.003 in) between the shoe and the drum, at the toe.
6.3.3 Filling and bleeding the brake system. The assembled simulated brake system
shall be filled with the fluid under test by means of a pressure pot, or by hand. If filled by hand
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