SD-03-2010

®

Bendix® PR-2™, PR-3™ & PR-4™ Pressure Protection Valves
Spring

Cap

Cap

Piston

Piston

Inlet
Valve

PR-3 or PR-4
Pressure Protection
Valves

Spring

Check
Valve

Supply
Port

Inlet
Valve

Supply
Port

Delivery
Port

Delivery
Port
Bendix® PR-3™
Pressure Protection Valve

Bendix® PR-4™
Pressure Protection Valve

Pressure
Regulating
Spring
Piston Plug

Exhaust
Vent

Piston Plug O-Ring

Piston O-Ring
Supply
Port

PR-2 Pressure
Protection Valve

Valve Stem

Piston

Inlet
Valve Spring

Spring
Retainer
Inlet
Valve

Delivery
Port
Bendix® PR-2™ Pressure Protection Valve
Figure 1 – Bendix® PR-2™, PR-3™ & PR-4™ Pressure Protection Valves

DESCRIPTION
The pressure protection valve is a normally closed,
pressure control valve which can be referred to as a
non‑exhausting sequence valve. These valves are used in
many different applications. An example would be in an air
brake system to protect one reservoir, or reservoir system
from another, by closing automatically at a preset pressure
should a reservoir system failure occur. The valves can also

be used to delay filling of auxiliary reservoirs to ensure a
quick build‑up of brake system pressure.
The Bendix® PR‑2™ and PR‑4™ pressure protection valves
have one 1/4” N.P.T. supply port and one 1/4” N.P.T. delivery
port which are identified. Both valves are provided with
two 9/32” mounting holes through the body. The closing
pressure of the PR‑2 valve is externally adjustable while
the PR‑4 valve has a fixed setting.
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GENERAL SAFETY GUIDELINES
WARNING! PLEASE READ AND FOLLOW THESE INSTRUCTIONS
TO AVOID PERSONAL INJURY OR DEATH:
When working on or around a vehicle, the following guidelines should be observed AT ALL TIMES:
▲ Park the vehicle on a level surface, apply the
parking brakes and always block the wheels.
Always wear personal protection equipment.
▲ Stop the engine and remove the ignition key
when working under or around the vehicle.
When working in the engine compartment,
the engine should be shut off and the ignition
key should be removed. Where circumstances
require that the engine be in operation, EXTREME
CAUTION should be used to prevent personal
injury resulting from contact with moving,
rotating, leaking, heated or electrically-charged
components.
▲ Do not attempt to install, remove, disassemble
or assemble a component until you have read,
and thoroughly understand, the recommended
procedures. Use only the proper tools and
observe all precautions pertaining to use of those
tools.
▲ If the work is being performed on the vehicle’s
air brake system, or any auxiliary pressurized air
systems, make certain to drain the air pressure
from all reservoirs before beginning ANY work
on the vehicle. If the vehicle is equipped with a
Bendix® AD-IS® air dryer system, a Bendix® DRM™
dryer reservoir module, or a Bendix® AD-9si® air
dryer, be sure to drain the purge reservoir.
▲ F o l l o w i n g t h e v e h i c l e m a n u f a c t u r e r ’s
recommended procedures, deactivate the
electrical system in a manner that safely removes
all electrical power from the vehicle.
▲ Never exceed manufacturer’s recommended
pressures.

▲ Never connect or disconnect a hose or line
containing pressure; it may whip and/or cause
hazardous airborne dust and dirt particles. Wear
eye protection. Slowly open connections with
care, and verify that no pressure is present. Never
remove a component or plug unless you are
certain all system pressure has been depleted.
▲ Use only genuine Bendix ® brand replacement
parts, components and kits. Replacement
hardware, tubing, hose, fittings, wiring, etc. must
be of equivalent size, type and strength as original
equipment and be designed specifically for such
applications and systems.
▲ Components with stripped threads or damaged
parts should be replaced rather than repaired.
Do not attempt repairs requiring machining or
welding unless specifically stated and approved
by the vehicle and component manufacturer.
▲ Prior to returning the vehicle to service, make
certain all components and systems are restored
to their proper operating condition.
▲ For vehicles with Automatic Traction Control
(ATC), the ATC function must be disabled (ATC
indicator lamp should be ON) prior to performing
any vehicle maintenance where one or more
wheels on a drive axle are lifted off the ground
and moving.
▲ The power MUST be temporarily disconnected
from the radar sensor whenever any tests USING
A DYNAMOMETER are conducted on a vehicle
equipped with a Bendix® Wingman® system.

▲ You should consult the vehicle manufacturer's operating and service manuals, and any related literature,
in conjunction with the Guidelines above.

OPERATION
Air entering the supply port is blocked from flowing out
the delivery port by the inlet valve, which is normally held
closed by the pressure regulating spring above the piston.
When sufficient air pressure builds beneath the piston, the
piston will move, opening the inlet valve and allowing air
to flow out the delivery port. As long as air pressure at the
supply port remains above the specified closing pressure,
the inlet valve will remain open.

Note:

The Bendix ® PR‑2 ™ & PR‑4 ™ valves 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 valve - Opening pressure 15‑20 psi higher
than closing pressure.
Bendix® PR‑3™ & PR‑4 valves - Opening pressure
approximately 10 psi higher than closing pressure.
PR‑3 valve - Check valve will retain maximum
pressure in downstream reservoir.

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If system air pressure decreases below the specified
closing pressure, the regulating spring will close the inlet
valve. The remaining air pressure, at either the supply
or delivery side, (depending on where the pressure drop
occurs) will be retained.

PREVENTIVE MAINTENANCE

IMPORTANT
Review the Bendix Warranty Policy before performing any
intrusive maintenance procedures. A warranty may be
voided if intrusive maintenance is performed during the
warranty period.
No two vehicles operate under identical conditions, as
a result, maintenance intervals may vary. Experience is
a valuable guide in determining the best maintenance
interval for air brake system components. At a minimum,
the Bendix® PR‑2™, PR‑3™, and PR‑4™ valves should
be inspected every 6 months or 1500 operating hours,
whichever occurs first, for proper operation. Should the
PR valves not meet the elements of the operational tests
noted in this document, further investigation and service
of the valve may be required.

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. Charge the air system to full pressure and shut off the
engine.
3. Observing the gauges on the supply and delivery sides
of the valve, slowly 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 valve only) Build pressure up again and shut the
engine off. Slowly exhaust air from the supply side of
the PR‑3 valve. The gauge on the delivery side of the
valve should remain at the highest pressure previously
attained.

LEAKAGE CHECKS
1. Charge the air system to full pressure and shut the
engine off.
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 valve ‑ No leakage is
permissible at the bottom of the 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 function as
described, or leakage is excessive, it is recommended that
it be replaced with a new or remanufactured unit or repaired
with genuine Bendix® parts available at Bendix outlets.

REMOVING AND INSTALLING
REMOVING
1. Block or hold the vehicle by means other than the air
brakes.
2. Drain all system reservoirs 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 valve 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 valves
are not adjustable.

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SD-03-2010 Rev. 008 © 2018 Bendix Commercial Vehicle Systems LLC, a member of the Knorr-Bremse Group • 10/18 • All Rights Reserved

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