Bendix Commercial Vehicle Systems DUAL RELAY VALVE User Manual

Bendix® Dual Relay Valve

SD-03-1000

PRIMARY
CONTROL
PORT (41)

DELIVERY

PORT (22)

RESERVOIR

SUPPLY

PORT (11)

MOUNTING

BRACKET STUDS

SECONDARY

CONTROL
PORT (42)

CAST PORT

IDENTIFICATION

FIGURE 1 - DUAL RELAY VALVE

DESCRIPTION

The Dual Relay Valve is a special purpose, multifunction
relay valve. It is typically used on articulated vehicles with
three or more axles. Normally mounted at the rear of the
vehicle, in proximity to the valves and chambers served, its
function is similar to a pilot relay and speeds the application
and release of control air pressure to the service relay
valve(s) on the rear most axle or axles of the vehicle. The
Dual Relay V alve is a pilot relay valve that responds to the
control air delivered to it from both circuits of the foot brake
valve. The Dual Relay Valve is controlled independently
by the front (secondary) and mid (primary) delivery circuits
of the dual circuit foot brake valve.

The Dual Relay Valve is supplied air from the rear axle(s)
service reservoir that is isolated from the other service
reservoirs forming a third service brake circuit on the
vehicle. With the addition of a third brake circuit the vehicle
will maintain braking on at least two axles in the event of a
single service system malfunction. In systems where this
valve is used, the brake circuits are designated as Front
(steering) Axle, Mid-Axle and Rear Axle.

RESERVOIR

SUPPLY

PORT (11)

EXHAUST

PORT (1)

Although typically nipple mounted to the rear service
reservoir the valve can be bracket mounted. Only four (4)
ports are utilized in the die cast body of the Dual Relay
V alve. All air connections are identifi ed with raised numerals
with the following designations:

Port ID Size Designation
41 3/8 NPTF Primary Control Port
42 3/8 NPTF Secondary Control Port
12, 43 Plugged Not used
22 3/8 NPTF Delivery
11 3/8 NPTF Rear Service Reservoir

OPERATION

Refer to fi gures 1 and 2. Each service reservoir is isolated
and is protected from loss of air in other service reservoirs.
The front and mid-axle service reservoirs supply air directly
to the secondary and primary circuits of the foot brake
valve, as well as antilock and traction components for their
respective circuits. The rear axle service reservoir supplies
air to the Dual Relay V alve, as well as the rear axle service
antilock relay and traction components.

1

™

BVA-85

DOOR

INTERLOCK

VALVE WITH

CONTROL

MODUL E

FAM

FRONT AXLE

MODUL E

™

PP-D C

PUSH-PULL VALVE WITH

INTEGRAL DOUBLE CHEC K

Secondary

Control

TO ACCESSORIES

DUAL

RELAY VALVE

REAR

Primary
Control

ABS-6

WITH

TRACTION CONTROL

RA M

REAR AXLE MODULE

AD-IS EP

EXTENDED PURGE

AIR DRYER

®

BA-922

TWIN CYLINDER

COMPRESSO R

FRONT

MID

FIGURE 2 - TYPICAL ARTICULATED COACH SCHEMATIC

System Normal

With the system normal and air pressure available in all
service reservoirs, brake application pressure is transmitted
from the primary and secondary circuits of the foot brake
valve.

Application pressure is transmitted simultaneously to the
front and mid-axle brakes and to the primary and secondary
control ports of the Dual Relay V alve. The Dual Relay V alve
applies or releases the rear axle brakes in direct proportion
to the control signals received from the foot brake valve.
The brake on each axle are applied using pressure from
their dedicated respective reservoirs.

Loss of Front Axle Service Reservoir

A service application made with a loss of air in the
secondary circuit results in a no or low application of the
front axle brakes. Air pressure from the primary circuit of
the foot brake valve applies the mid-axle brakes and enters
the primary control port of the Dual Relay V alve. The Dual
Relay Valve responds to primary control and applies the
rear axle brakes. Brake release occurs when foot brake
valve pressure is removed.

Loss of Mid-Axle Service Reservoir

A service application made with a loss of air in the primary
circuit results in a no or low application of the mid-axle
brakes. Air pressure from the secondary circuit of the foot
brake valve applies the front axle brakes and enters the
secondary control port of the Dual Relay Valve. The Dual
Relay V alve responds to secondary control and applies the
rear axle brakes. Brake release occurs when foot brake
valve pressure is removed.

Loss of Rear Axle Service Reservoir

With a loss of rear axle service pressure, the front and
mid-axle brakes will be applied by the foot brake valve.
Primary and secondary control pressure is received by the
Dual Relay Valve however, a no or low application of the
rear brakes will occur due to a loss of pressure in the rear
service reservoir. Front and mid-axle brakes will release
when foot brake valve pressure is removed.

PREVENTIVE MAINTENANCE

GENERAL

Important: Preventive maintenance for the Dual Relay
Valve consists entirely of inspections and tests. The Dual
Relay V alve is not serviceable and must be replaced when
testing or inspection warrants it. Kits and replacement parts
are not available.

Because no two vehicles operate under identical
conditions, maintenance and maintenance intervals will
vary . Experience is a valuable guide in determining the best
maintenance interval for any one particular operation.

Perform the tests and inspections presented at the
prescribed intervals. If the Dual Relay V alve fails to function
as described or leakage is excessive, it must be replaced
with a genuine Bendix unit, available at any authorized
parts outlet.

Every 3 months, or 25,000 miles or 900 operating
hours:

1. Remove any accumulated contaminants and visually
inspect the exterior for excessive corrosion and
physical damage.

2