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