BENDIX SD-03-1068 User Manual

Bendix® R-12DC™ Relay Valve with Biased Double Check

PRIMARY

SERVICE

SUPPLY (2)

EXTERIOR R-12DC

(MODEL WITH 2 HORIZONTAL PORTS)

FIGURE 1 - EXTERIOR VIEWS

™

VALVE

SECONDARY

SERVICE

DELIVERY

EXTERIOR

R-12DC™ VALVE

(MODEL WITH

4 VERTICAL

DELIVERY PORTS)

EXTERIOR
R-12DC™ VALVE
(MODEL WITH 2

HORIZONTAL AND 2

VERTICAL DELIVERY

PORTS)

SD-03-1068

DESCRIPTION

The relay valve in an air brake system functions as a relay
station to speed up the application and release of the
brakes. The valve is normally mounted at the rear of the
vehicle in proximity to the chambers it serves. The valve
operates as a remote controlled brake valve that delivers

PRIMARY
SERVICE

CAP SCREW

SEALING

RING

RELAY
PISTON
O-RING

EXHAUST

SEAT

SPRING

O-RINGS

RETAINING

RING

FIGURE 2 - R-12DC™ RELAY VALVE WITH BIASED DOUBLE CHECK SECTIONAL VIEW

PORT

COVER

RELAY

PISTON

VALVE

RETAINER

EXHAUST

EXHAUST

PORT

or releases air to the chambers in response to the control
air delivered to it from the foot brake valve.

The R-12DC™ relay valves are designed for either reservoir
or frame mounting. (See Figure 1). For ease of servicing,
the inlet/exhaust valve can be replaced without the need for
line removal.

SECONDARY

SERVICE PORT

INLET

VALVE

SPRING

DOUBLE CHECK

DIAPHRAGM

GUIDE

1

OPERATION

APPLICA TION

Under normal conditions, the internal biased double check
valve assures that the primary service signal controls the
valve. Air pressure delivered to the primary service port
enters the small cavity above the piston and moves the
piston down. The exhaust seat moves down with the piston
and seats on the inner or exhaust portion of the inlet/exhaust
valve, sealing off the exhaust passage. At the same time,
the outer or inlet portion of the inlet/exhaust valve moves
off its seat, permitting supply air to flow from the reservoir,
past the open inlet valve and into the service brake
chambers. In the event of a loss of the primary service
line, (see Figure 4) the double check valve mechanism in
the cover of the R-12DC™ valve will move, shutting off the
primary service line, and instead allow the secondary service
line to apply the air pressure needed to operate the valve.

Note: Secondary service line may leak out of the primary
service at control pressures up to 20 psi when the primary
signal is not present.

BALANCE

The air pressure being delivered by the open inlet valve
also is effective on the bottom area of the relay piston.

™

R-12DC

VALVE

RELAY PISTON

INLET

EXHAUST

VALVE

RESERVOIR

ANTILOCK SENSOR

TONE RING

FIGURE 3 - R-12DC
SYSTEM COMPONENTS

™

SECTIONAL VIEW WITH TYPICAL

BRAKE VALVE

(DELIVERY
LINES ARE

SHOWN)

ANTILOCK

MODULATOR

EXHAUST

PORT

BRAKE

CHAMBERS

BRAKES

APPLIED

BRAKE PEDAL

APPLIED

BRAKE PEDAL

APPLIED

NORMAL: BIASED

DOUBLE-CHECK VALVE

ALLOWS PRIMARY

CONTROL LINE SUPPLIES

RELAY PISTON

CONTROL LINE

FAILURE

BRAKE PEDAL

APPLIED

SECONDARY

CONTROL LINE

SUPPLIES RELAY

PISTON

FIGURE 4 - R-12DC

2

™

APPLIED POSITION (SHOWING BIASED DOUBLE CHECK OPERATIONAL VIEWS)

BRAKE PEDAL

POSITION

HELD

BRAKE

CHAMBERS

MAINTAIN

APPLICATION

AIR PRESSURE

FROM BRAKE

CHAMBERS IS

EXHAUSTED

HERE

BRAKE
PEDAL

RELEASED

AIR PRESSURE

ABOVE RELAY

VALVE IS

EXHAUSTED

HERE

BRAKE
CHAMBERS
RELEASED

FIGURE 5 - R-12DC

™

BALANCE POSITION

When air pressure beneath the piston equals the service air
pressure above, the piston lifts slightly and the inlet spring
returns the inlet valve to its seat. The exhaust remains
closed as the service line pressure balances the delivery
pressure. As delivered air pressure is changed, the valve
reacts instantly to the change, holding the brake application
at that level.

EXHAUST OR RELEASE

When air pressure is released from the service port and air
pressure in the cavity above the relay piston is exhausted
through the brake valve. At the same time, air pressure
beneath the piston lifts the relay piston and the exhaust
seat moves away from the exhaust valve, opening the
exhaust passage. With the exhaust passage open, the air
pressure in the brake chambers is then permitted to exhaust
through the exhaust port, releasing the brakes.

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

FIGURE 6 - R-12DC

™

EXHAUST POSITION

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 first, 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.

REMOVAL AND INSTALLATION

REMOVAL

1. Block and hold vehicle by means other than air brakes.

2. Drain air brake system reservoirs.

3. If entire valve is to be removed, identify air lines to facilitate
installation. Prior to disassembly, remove as much
contamination as possible from the exterior of the device
taking care to keep all contamination from entering the
open ports.

4. Disconnect air lines from valve*.

5. Remove valve from reservoir or if remotely mounted,
remove mounting bolts and then valve.

*It is generally not necessary to remove entire valve to service
the inlet/exhaust valve. The inlet/exhaust valve insert can be
removed by removing the snap ring, exhaust cover assembly
and then inlet/exhaust valve.

3