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OPERATION
Air entering the supply port is initially prevented from flowing
out the delivery port by the inlet valve which is held closed
by the pressure regulating spring above the piston. When
sufficient air pressure builds beneath the piston to overcome
the setting of the regulating spring, the piston will move,
causing the inlet valve to unseat (open), and allow air to
flow out the delivery port. As long as air pressure at the
supply port and beneath the piston remains above the
specified closing pressure, the inlet valve will remain open.
NOTE: The PR-2 and PR-4 closing pressure is noted on
the label affixed to the valve. Opening pressures of
the valves are higher than closing pressures. The
pressure ranges are noted below:
PR-2-Opening pressure 15-20 psi higher than closing
pressure.
PR-3 & PR-4-Opening pressure approx. 10 psi higher
than closing pressure.
PR-3-Check valve will retain maximum pressure in
downstream reservoir.
If for any reason system air pressure is decreased below
the specified closing pressure, the regulating spring will
move the piston closing the inlet valve. The remaining air
pressure at either the supply or delivery side, (depending
upon where the pressure drop has occurred) will be retained.
PREVENTIVE MAINTENANCE
Every three months, 900 operating hours or 25,000 miles,
whichever if first, it is recommended that the operation and
leakage checks described in this manual be performed.
OPERATING AND LEAKAGE CHECKS
OPERATING CHECKS
1. Provide a pressure gauge and drain valve at the supply
side and delivery side of the pressure protection valve
being checked.
2. Build up the air system to full pressure and shut off the
engine.
3. While watching the gauges on the supply and delivery
sides of the valve, slowly begin to exhaust pressure from
the delivery side. Note that both gauges will show
pressure loss until the closing pressure of the pressure
protection valve is reached.
The pressure protection valve should close at
approximately ( ± 5 psi) the pressure indicated on the
valve’s label or in the vehicle handbook. The gauge on
the delivery side of the valve should continue to show
loss of pressure while the gauge on the supply side
should stop at the same pressure as the setting of the
valve.
4. (PR-3 only) Build pressure up again and shut off engine.
Slowly exhaust air from the supply side of the PR-3.
The gauge on the delivery side of the valve should
remain at the highest pressure previously attained.
LEAKAGE CHECKS
1 . Build up the air system to full pressure and shut off the
engine.
2. Apply a soap solution around the cap of the pressure
protection valve. A one-inch bubble in three seconds or
longer is acceptable. PR-3 - No leakage permissable at
bottom of valve.
3. Drain the air pressure from the delivery side of the
pressure protection valve and disconnect the air line to
it.
4. Apply a soap solution to the delivery port. A one inch
bubble in five seconds or more is acceptable.
GENERAL
If the pressure protection valve does not operate as
described or leakage is excessive, it is recommended that
a replacement be obtained at the nearest authorized
AlliedSignal Truck Brake Systems Co. distributor.
REMOVING AND INSTALLING
REMOVING
1 . Block or hold the vehicle by means other than air
brakes.
2. Drain all system reservoirs individually, to 0 psi.
3. Disconnect and identify (supply and delivery) the air
lines leading to and from the pressure protection valve.
4. Remove the mounting bolts, if any, that secure the valve.
INSTALLING
1. Re-install the mounting bolts and secure the replacement
valve to the vehicle.
2. Reconnect the supply delivery air lines to the proper
ports of the replacement valve.
GENERAL
After installing a replacement valve, it is recommended that
the operating and leakage checks be performed as outlined
in this manual. If the closing pressure does not conform to
that shown on the valve label or in the vehicle or a different
setting is desired, the PR-2 may be adjusted by loosening
the locknut and tightening or loosening the adjusting cap as
required; however, if the proper setting cannot be attained
by moderate adjustment of the cap, the valve may have the
wrong spring and will have to be exchanged for the correct
valve. The PR-3 and PR-4 are not adjustable.
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