BENDIX R-12P User Manual

Bendix® R-12P™ Pilot Relay Valve

SD-03-1063

1/4” CONTROL

COVER

1/2” SUPPLY (2)

FIGURE 1 - CUTAWAY AND EXTERNAL VIEW

3/8” DELIVERY (4)

BODY

1/2”

SUPPLY

RELAY

PISTON

INLET/EXHAUST

VALVE

RESERVE

SPRING

LOAD

PISTON

1/4” CONTROL

EQUALIZING

ORIFICE

3/8”

DELIVERY

EXHAUST

DESCRIPTION

The R-12P™ pilot relay valve is designed to speed up
control signal application and release on long non-towing
trailers, towing trailers, dollies, and long wheelbase tractors.
It does this without adversely affecting the control signal.
The R-12P™ valve replaces the current R-8P™ valve.

Unlike a standard relay valve used as a “booster,” which
typically has a 2-4 psi differential between control and
delivery , the R-12P™ valve has zero differential. It “speeds”
up control line signal application and release in a train
of vehicles, and it delivers the same air pressure that it
receives. This provides a uniform brake signal to all trailer
and dolly brakes, even during light applications.

The unique R-12P™ valve feature is that it uses a supply
air pressure load to quicken the valve’s response time. By
balancing the pressure signal to all trailer and dolly brakes,
the R-12P™ valve aids in achieving overall vehicle brake
pressure balance, timing balance, and stability.

It also substantially improves brake lining wear balance
on combination vehicles by making even the most distant
brake contribute its share of braking effort.

Figure 2 shows the R-12P™ valve in a trailer air brake
schematic. Note that additional air lines for a towing trailer
are shown as dashed lines.

The R-12P™ valve uses the R-12™ valve family lower body
and standard Bendix mounting brackets. It can also be
reservoir-mounted.

The body and cover are die-cast aluminum, and the valve’s
weight is approximately 2.3 lbs. Porting is as follows:

Qty. Size
Supply (SUP) 2 1/2”
Delivery (DEL) 4 3/8”
Control (CON) 1 1/4”
Exhaust (EXH) 1 --

1

TRAILER BRAKE SYSTEM WITH R-12P™ PILOT RELAY VALVE

VENTED

DELIVERY

CONTROL

CUT-OUT

COCK

CONTROL

SUPPLY

™

R-12P

VALVE

SUPPLY

SINGLE

CHECK VALVE

SPRING

BRAKE

VALVE

RESERVOIR

SLACK ADJUSTER

-- -- -- -- -- ADDITIONAL PLUMBING FOR TOWING TRAILER

SERVICE RELAY

VALVE

BRAKE CHAMBER

QUICK

RELEASE

VALVE

VENTED

CUT-OUT

COCK

FIGURE 2 - TYPICAL R-12P™ PILOT RELAY VALVE SYSTEM SCHEMATIC

OPERATION

CHARGING

When the air brake system is charging, the relay pis ton’s
exhaust seat is against the inlet/exhaust valve due to
the force of the reserve spring. Reservoir air enters the
R-12P™ valve supply port and becomes active upon the
inlet/exhaust valve. At the same time, reservoir air passes
through the R-12P™ valve body channel to the load piston.
Supply air builds simultaneously and equally underneath the
inlet/exhaust valve and above the load piston. While supply
pressure continues to hold the inlet/exhaust valve against

2

its seat, it also moves the load piston into contact with the
relay piston. However, even though force is increasing on
the relay piston, it does not move the inlet/exhaust valve.
The effective area of the inlet/exhaust valve is larger than
the effec tive area of the load piston. Therefore, load piston
force cannot overcome inlet/exhaust valve force. So supply
air builds to governor cut-out and the inlet/exhaust valve
remains closed. This is the position of the R-12P™ valve
in normal run mode, as shown in Figure 3.

LOAD PISTON

RELAY PISTON

SUPPLY PORT

INLET/EXHAUST

VALVE

FIGURE 3 - CHARGING

CONTROL

PORT

DELIVERY

PORT

Note that these events occur quickly , due to the “preload”
on the relay piston. This preload, provided by supply air
pressure acting on the load piston, is a feature unique to
the R-12P

™

valve.

BALANCE

Air pressure being delivered by the open inlet valve is
also effective underneath the R-12P™ valve relay piston.
When air pressure beneath the piston approaches control
pressure on top of the piston, the piston and inlet valve lift
together and close the inlet valve. The exhaust remains
closed, and the valve achieves “force balance.” That is,
the piston and inlet/exhaust valve are in their neutral
state. This is the position commonly known as “balanced”
when referring to conventional relay valves. However,
conventional relay valves cannot achieve true air pressure
balance because of their inherent differentials. The R-12P™
valve achieves this balance because of its equalizing
orifi ce. The orifi ce allows control pressure and delivery
pressure to be “common” and equalize. In this position,
shown in Figure 5, sustained service brake applications
can be held, with no “residual” differential.

LOAD PISTON

RELAY PISTON

SUPPLY PORT

INLET/EXHAUST

VALVE

FIGURE 4 - NORMAL (RAPID) APPLICATION

CONTROL

PORT

NORMAL (RAPID) APPLICATION

Figure 4 shows an R-12P™ valve receiving a “normal”
service brake application. “Normal” means the control
signal is applied rapidly enough to build up pressure on
the relay piston and “open” the inlet valve, causing the
R-12P™ valve to react like a relay valve. The relay piston
opens the inlet, allowing supply air to fl ow out the delivery
port to the next valve in the system.

EQUALIZING

ORIFICE

PROVIDES AIR

PRESSURE

BALANCE

SUPPLY PORT

INLET/EXHAUST

VALVE

FIGURE 5 - BALANCE

LOAD PISTON

RELAY

PISTON

CONTROL PORT

DELIVERY

PORT

3

RELAY PISTON

LOAD PISTON

CONTROL PORT

LOAD PISTON

RELAY PISTON

CONTROL PORT

SUPPLY PORT

INLET/EXHAUST

VALVE

FIGURE 6 - RAPID EXHAUST FIGURE 7 - EQUALIZING (SLOW) APPLICATION

DELIVERY PORT

EXHAUST

SUPPLY PORT

INLET/EXHAUST

VALVE

DELIVERY PORT

RAPID EXHAUST

Figure 6 shows a rapid service brake release. Air above
the relay piston travels back out the control port to be
exhausted. With the lack of air pressure above the
piston, air pressure below lifts the piston, closing the inlet
and opening the exhaust. Delivery air then exhausts to
atmosphere.

EQUALIZING (SLOW) APPLICATION

When a control signal enters the R-12P™ valve, it acts
on the relay piston. However, Figure 7 shows the control
signal passing through the valve without causing piston
movement. The air is passing through the relay piston’s
equalizing orifi ce, directly through the valve and out the
delivery port to the next valve in the air system. This
situation occurs if air pressure build-up is slow and the
pressure above the relay piston is not strong enough to
open the valve. (Air pressure above the piston must be at
least 3 psi greater than air pressure below the piston to
cause movement.)

For example, if a vehicle is traveling over a long downhill
grade, the driver may want to limit speed and maintain
vehicle control through proper gearing, engine speed and
a light service brake application.

LOAD PISTON

RELAY PISTON

CONTROL PORT

In the equalizing application, even though the inlet valve
does not open, the control signal is not decreased because
the equalizing orifi ce allows it to pass directly through the
R-12P™ valve. Note that this condition exists during very
slow service brake applications.

4

SUPPLY PORT

INLET/EXHAUST

VALVE

FIGURE 8 - EQUALIZING (SLOW) EXHAUST

DELIVERY PORT

If, during the light brake application, the driver modulates
the treadle slowly , and not enough to open the R-12P™ inlet
valve, the desired control pressure will still pass on to the
next valve, maintaining zero differential.

EQUALIZING (SLOW) EXHAUST (SEE FIGURE 8)

RELAY PISTON

LOAD PISTON

RESERVE SPRING

SEALS EXHAUST

As described earlier, the result of any service brake
application with an R-12P™ valve is air pressure balance
above and below the relay piston. If the brake valve is
modulated slowly upon release so that pressure beneath
the piston is not great enough to open the exhaust, delivery
air will pass through the equalizing orifi ce and out the
control port to be exhausted.

REDUCTION OF SUPPLY PRESSURE

Figure 9 shows the R-12P™ valve with no supply pressure.
With no supply air, the reserve spring is strong enough to
hold the relay piston on the exhaust seat, but it is not strong
enough to overcome the force of the inlet/exhaust valve
spring. So both inlet and exhaust valves are closed.

Service Brake Applications with Reduced Supply
Pressure

Rapid Application - Use of Supply Line Single Check
Valve

If a rapid service brake application is made, control air moves
the relay piston and opens the inlet valve; but there is no
supply air to deliver. Control air then travels into the supply
port. The single check valve in the R-12P™ valve supply
port (shown in Figure 2) prevents this air from escaping to
atmosphere through the open or damaged supply source.
Control air also passes through the equalizing orifi ce and
out the delivery port, assuring no reduction or loss of control
signal to subsequent vehicles.

Equalizing (Slow) Application

If an equalizing (slow) service brake application is made
with reduced supply air pressure, control air passes
through the equalizing orifi ce and out the delivery port to
subsequent vehicles in the train.

Control air cannot escape to atmosphere because the
reserve spring keeps the exhaust closed.

CONTROL

PORT

SUPPLY PORT

INLET/EXHAUST

VALVE

FIGURE 9 - REDUCTION OF SUPPLY PRESSURE

DELIVERY PORT

WARNING! PLEASE READ AND FOLLOW
THESE INSTRUCTIONS TO A VOID PERSONAL
INJURY OR DEATH:

When working on or around a vehicle, the following
general precautions should be observed at all times.

1. Park the vehicle on a level surface, apply the
parking brakes, and always block the wheels.
Always wear safety glasses.

2. Stop the engine and remove 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.

3. 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.

4. 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 an
AD-IS® air dryer system or a dryer reservoir module,
be sure to drain the purge reservoir.

5

5. Following the vehicle manufacturer’s recommended
procedures, deactivate the electrical system in a
manner that safely removes all electrical power
from the vehicle.

6. Never exceed manufacturer’s recommended
pressures.

7. Never connect or disconnect a hose or line
containing pressure; it may whip. Never remove
a component or plug unless you are certain all
system pressure has been depleted.

®

8. Use only genuine Bendix

replacement parts,
components and kits. Replacement hardware,
tubing, hose, fi ttings, etc. must be of equivalent
size, type and strength as original equipment and
be designed specifi cally for such applications and
systems.

9. Components with stripped threads or damaged
parts should be replaced rather than repaired. Do
not attempt repairs requiring machining or welding
unless specifi cally stated and approved by the
vehicle and component manufacturer.

10. Prior to returning the vehicle to service, make
certain all components and systems are restored
to their proper operating condition.

11. For vehicles with Antilock 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.

SERVICE CHECKS

1. Remove any accumulated contaminants. Visually
inspect the valve’s exterior for excessive corrosion
or physical damage. Repair/replace the valve as
necessary.

2. Inspect all air lines connected to the valve for signs
of wear or physical damage. Repair/replace as
necessary.

3. Test air line fi ttings for excessive leakage and tighten
or replace as necessary.

OPERATIONAL AND LEAKAGE TESTS

1. Block the vehicle’s wheels and fully charge the air
system.

2. Apply and release the service brakes several times
and check for prompt response of the brakes at all
appropriate wheels.

3. With the air system fully charged, apply a soap solution
to the R-12P™ valve exhaust port. Leakage of a 1”
bubble in 5 seconds is permissible.

4. Make and hold a full brake application and again apply
a soap solution to the R-12P™ valve exhaust. Leakage
of a 1” bubble in 3 seconds is permissible.

5. With the brakes still applied, apply a soap solution
around the valve where the cover meets the body. No
leakage at this point is permitted.

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 valve
should be inspected every 6 months or 1500 operating
hours, whichever comes fi rst, for proper operation. Should
the valve not meet the elements of the operational tests
noted in this document, further investigation and service
of the valve may be required.

If the valve does not function as described; or if leakage is
excessive, repair the valve or replace it at any authorized
parts outlet.

NOTE: If a supply line single check valve is present (see

Figure 2), check it for leakage. Disconnect the inlet

side of the single check valve and coat the open
end with a soap solution. Make and hold a full
brake application. Leakage of a one inch bubble in
fi ve seconds is permissible. If the valve’s leakage
is excessive, replace it with a 1/2” single check
valve.

REMOVAL

1. Drain all system air pressure.

2. Identify and mark or label all air lines and their
connections to the R-12P™ valve and the single check
valve, if present. Then disconnect the air lines.

3. Remove the R-12P™ valve from the vehicle.

6

FASTENER

COVER

LOAD

PISTON

3

RELAY PISTON

4

1

2

RESERVE

SPRING

BODY

12

11

10

9

8

7

6

5

FIGURE 10 - EXPLODED VIEW

INSTALLATION

1. Use the mounting bracket provided, or, if securing the
valve to a reservoir, use a Schedule 80 (heavy wall)
short couple pipe nipple.

2. Reconnect all air lines to the valve using the
identifi cation made during removal. Be sure to use pipe
thread sealant.

3. T est all air fi ttings for excessive leakage and tighten as
needed. Also, perform OPERA TIONAL AND LEAKAGE
TESTS before placing the vehicle back into service.

DISASSEMBLY

The following procedure is for reference only. Always
have the appropriate maintenance kit on hand, and use
its instructions in lieu of those presented here. Refer to
Figure 10 throughout.

™

CAUTION: The R-12P

a bench vise during disassembly. However,
overclamping will cause damage to the valve
and result in leakage and/or malfunction. If a
vise is used, position the valve so the jaws
bear on the supply ports on opposing sides
of the valve’s body.

valve may be lightly clamped in

1. Remove the four fasteners that secure the cover to the
body. Then slowly remove the cover.

2. Remove and discard the small o-ring(1) that seals the
internal channel and the large o-ring(2) between the
cover and the body.

3. Remove the load piston from the cover. Note: If
necessary, use approximately 20 psi of shop air,
directed into the load piston’s air passage in the cover ,
to dislodge it. CAUTION: The piston will leave the cover
with some force.

4. Remove and discard the load piston’s o-ring(3).

5. Remove the reserve spring and the relay piston from the
body . Remove and discard the relay piston’s o-rings(4)
and (5).

6. While holding exhaust cover(7), remove and discard
the snap ring(6) that secures the cover to the body.

7. Remove and discard exhaust cover(7) and its o-rings(8)
and (9).

8. Remove and discard valve spring(10), valve retainer(11),
and inlet/exhaust valve(12) from the body.

7

CLEANING & INSPECTION

1. Using mineral spirits or an equivalent solvent, clean and
thoroughly dry all metal parts. Do not damage bores
with metal tools.

2. Wash all non-metallic components in a soap and water
solution. Dry thoroughly.

3. Inspect interior and exterior of all metal parts for severe
corrosion, pitting, and cracks. Superfi cial corrosion
and/or pitting on the exterior of the body and cover
is acceptable. Replace the entire valve if the body or
cover interior show signs of corrosion or pitting.

4. After washing, inspect relay piston for cracks, wear,
or distortion. Replace the valve if these conditions are
found.

5. Make certain the air channel running from the cover
through the top surface of the body to the supply port
is clear and free of obstruction.

6. Inspect the pipe threads in the body. Make certain they
are clean and free of thread sealant.

7. Inspect all air line fi ttings for corrosion. Replace as
necessary. Remove all old thread sealant before
reuse.

ASSEMBLY

1. Before assembly , lubricate all o-rings, seals, and pistons
as well as body and cover bores, using the lubricant
provided in the maintenance kit. Use all of the lubricant,
and spread it evenly on all rubbing surfaces.

2. Install valve retainer(11) on inlet/exhaust valve(12) so
that the fl ange of the retainer surrounds the rubber
portion of the valve.

3. Install inlet/exhaust valve(12) in the body.

4. Install valve spring(10) over the inlet/exhaust valve in
the body.

5. Install large and small diameter o-rings(8) and (9) in
exhaust cover(7).

6. Place the exhaust cover on the inlet/exhaust valve
spring. Then depress the cover against the spring’s
force into the body. Secure the cover with snap
ring(6).

7. Install o-rings(4) and (5) onto the relay piston. Then
place the reserve spring in the piston and install the
piston into the body.

8. Install the load piston’s o-ring(3) onto the load piston.
Then install the load piston into the cover, small end
fi rst.

9. Install small o-ring(1) that seals the internal channel
and large o-ring(2) onto the cover.

10. Place the cover on the body and secure with its four
fasteners. Torque to 120 - 150 in. lbs.

11. Perform OPERATIONAL AND LEAKAGE TESTS before
returning the vehicle to service.

8

BW1711 © 2007 Bendix Commercial Vehicle Systems LLC. All rights reserved. 7/2007 Printed in U.S.A.