When the towing vehicle air brake treadle is
(1) Remove snap ring (16), and then remove valve
guide (17), packing (18), and spring (19) from piston
depressed for brake application, air enters the service
port and acts against diaphragm (8) and plunger (9).
This action moves the plunger against exhaust valve
(2) Unscrew exhaust valve assembly (22) from
inlet valve assembly (20) and remove both parts.
(22), closes the exhaust port in the plunger, and un-
seats valve (20). With valve unseated, air from the
(3) Remove packings (21, 23, and 24).
g. Remove snap ring (26) and then remove plug (27),
trailer reservoir enters the relay valve and is directed
out the delivery ports to apply the brakes.
packing (28), spring (29), and check valve (30) from
relay valve body.
d. As the towing vehicle air brake treadle is
h. Disassemble relay valve body assembly by
released, the pressure from spring (11) forces the air
back through the service port line where it is
removing packing (31), plugs (32, 33, 35, and 36),
strainer (34), and bleed valve (37).
exhausted through the treadle valve. With the air
released above diaphragm (8), the exhaust port in the
8-3. Cleaning and Inspection
plunger opens. Spring pressure against the brake
chamber diaphragm causes the diaphragm to force
a. Clean all metal parts with dry cleaning solvent
the air back through the lines, and into the relay
(fed. spec. P-D-680) and dry thoroughly.
valve. The air exhausts through the top of the relay
b. Inspect castings for damage, cracks, warping,
and stripped or crossed threads.
e. During emergency operation of the relay valve,
C. Check springs for weakness or distortion.
when the trailer connections are severed, the trailer
d. Inspect diaphragms for cracks or deteriorated
reservoir pressure closes the ball check valves in the
hoses at the outlet side of the air cleaners. At the
e. Inspect seating surfaces of valves for damage.
same instant, reservoir air pressure, which is greater
f. Replace damaged parts. Always use new pre-
than the pressure of spring (19), overcomes the spring
formed packings and seal when reassembling.
and permits full reservoir pressure to enter the relay
valve. The pressure exits through the delivery ports
and applies the brakes.
relay valve in the reverse order of disassembly. Note
a. Remove brake relay valve as instructed in
(1) During reassembly you should apply a light
coating of oil on preformed packings (18, 21, 23, 24,
28, and 31) and the lip of the oil seal (10).
b. Remove screw (1, fig. 8-1), exhaust cap (2), and
(2) After reassembly, test relay valve prior to
diaphragm (3) from cover (5).
reinstalling on trailer. Proceed as follows:
c. Remove the cover assembly (items 5 through 10)
(a) Apply full air brake system pressure to sup-
by removing the eight screws (4) that secure it to the
ply port in top cover (5). Apply soap solution to
body (38). Disassemble the cover assembly by
delivery ports and check for bubbles. No leakage is
removing nut (6), diaphragm plate (7), diaphragm (8),
permitted. If bubbles appear, diaphragm (3) is faulty
plunger (9), and seal (10).
and requires replacement.
d. Remove spring (11).
(b) Apply system pressure to reservoir port in
e. Remove the baffle plate (13) from body (38) by
body (38). Apply soap solution to delivery ports,
removing three screws (12).
emergency port, and hole in end cap. No leakage is
f. Unscrew end cap (14) and remove spring (15) and
permitted. If bubbles appear at delivery port, the
inlet valve (20) is faulty. If there are bubbles at the
cartridge assembly (items 16 through 22) from relay
valve body. Do not disassemble the cartridge assem-
emergency port, the check valve is faulty, Bubbles at
bly unless a malfunction is indicated. When
the end cap indicate a faulty packing on the inlet
valve or on the piston (25), Replace valves or packings
necessary, disassemble the cartridge assembly as