MCi Crusader II 1989 Maintenance Manual page 395

Date
1-1-89
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MC-9 MAINTENANCE MANUAL
r'RESSURE PROTECTION VALVE
On the MC-9 the valve is located in the right hand front
baggage compartment above and behind the main water valve
(figure 12-23).
The pressure protection valve controls the pressure at
which compressed air is delivered to the system. The valve
remains closed until a pre-set supply pressure is reached (65
psi. 448 kPa). It then opens and passes air to the delivery port.
The chief use for this valve is to protect the main air system,
to control the amount of air taken from the main air brake system
to operate auxiliary devices. By locating the pressure protec-
tion valve between the dry air tank and park brake air tank. air is
maintained in the main air brake system before being supplied
to the park brake air tank for quick build-up for emergency
applications.
The pressure protection valve has a ¼" (6.35 mm) delivery
port on the bottom and a
'/4''
(6.35 -mm) supply port on the side.
Two 9132" (7.15 mm) mounting holes are cast into the body for
mounting purposes if required. See figure 12-24.
OPERATION
Compressed air enters the supply port. and is stopped by the
closed inlet valve. When the air pressure entering the supply
ADJUSTING NUT
port overcomes the pressure setting of the spring above the
piston. the piston moves upward. The inlet valve opens. allow-
ing air to pass out the delivery port.
When air pressure underneath the piston is reduced to a
lesser force than that of the spring above it, the spring forces the
piston down. closing the inlet valve and stopping further flow of
air through the delivery port of the valve.
The pressure setting is determined by the force of the spring
which is controlled by the cap on the top of the valve. Turning
the cap clockwise raises the pressure setting; turning the cap
counterclockwise lowers the pressure setting. The lock nut
must be tightened after each adjustment
MAINTENANCE
Every 6 months or 30,000 miles (48,000 km), connect a test
gauge to the delivery line and check the pressure setting by
observing at what pressure valve closes by venting the delivery
system. ·1f the pressure setting varies more than 5 psi (34 kPa)
from the specified setting (65 psi, 448 kPa). adjust the valve.
(See
Operating and Leakage Checks.) Tighten lock nut after
each adjustment
BODY
Figure 12-24. Pressure Protection Valve Parts Breakdown.
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