BENDIX SD-03-2202 User Manual
Bendix® DC-4® Double Check Valve
SD-03-2202
Frame MountIn-line Mount
SUPPLY
DELIVERY
O-RING
CAP
BODY
FIGURE 1 - BENDIX® DC-4® DOUBLE CHECK VALVE
SUPPLY
SHUTTLE
VALV E
SHUTTLE
GUIDE
MOUNTING
SUPPLY
HOLE
SUPPLY
DELIVERY
DESCRIPTION
The Bendix® DC-4® double check valve is used in
an air brake system to direct a flow of air into a
common line from either of two sources, whichever is
at the higher pressure. The valves may be used for
directing air ow for specic functions or to select the
higher pressure of either of two sources of air as a
supply source.
Bendix manufactures two double check valves; one is
mounted in the air line without being secured to the vehicle
frame, and the other contains a mounting tab to secure
it to the vehicle frame or chassis. Although the valves
are somewhat different physically, the same function is
performed by each type.
OPERATION
As air under pressure enters either end of the double
check valve (inlet port), the moving shuttle responds to the
pressure and seals the opposite port—assuming it is at a
lower pressure level than the other. The air ow continues
out the delivery port of the double check valve. The position
of the shuttle will reverse if the pressure levels are reversed.
Double check valves are designed so that the shuttle can
never impede the backow of air in the exhaust mode.
Figure 2 (see page 2) illustrates a typical double check
valve used to control a given device—such as trailer
brakes—from either of the two control sources.
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PREVENTIVE MAINTENANCE
Reservoir
Brake Valve
To Trailer
Brakes
Trailer Hand
Control Valve
FIGURE 2 - DOUBLE CHECK VALVE: CONTROL OF
SYSTEM FROM EITHER OF TWO CONTROL SOURCES
Bendix®
PP-7™ Valve
Bendix®
PP-1®
Valve
Trailer Supply
Spring Brake Control
Reservoir
Reservoir
FIGURE 3 - DOUBLE CHECK VALVE: SYSTEM WITH TWO
SUPPLY SOURCES
Double
Check
Valve
Double Check
Valve
Figure 3 illustrates a typical double check valve used to
supply air to a system (or systems) from either of two
separate sources—whichever is at the greater pressure
level. In this type of installation, the pressure differential
to which the valve is subjected may—under certain
conditions—be minimal. As a result, performance of the
double check valve will be optimized if it is mounted in the
horizontal position.
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® DC-4® double check valve should be inspected
every 6 months or 1500 operating hours, whichever comes
rst, for proper operation. Should the DC-4 valve not
meet the elements of the operational tests noted in this
document, further investigation and service of the valve
may be required.
SERVICE CHECKS
OPERATING AND LEAKAGE TEST
A. When the double check valve is used in conjunction
with a trailer control valve, the following operating and
leakage test can be made:
1. Apply and release the foot brake valve and note that
the brakes apply and release on both the tractor
and the trailer.
2. Apply and release the trailer control valve and note
that only the trailer brakes apply and release. With
the trailer control valve applied, check the exhaust
port of the foot brake valve for leakage with a soap
solution. Permissible leakage is a 1 inch bubble in
5 seconds (100 sccm).
3. Apply and hold a full foot brake valve application.
Check the exhaust port of the trailer control valve for
leakage with a soap solution. Permissible leakage
is a 1” bubble in 5 seconds (100 sccm). Note: On
some vehicles, an exhaust line is connected to
the exhaust port and piped outside the cab. In this
case it may be necessary to disconnect this line to
complete the leakage check.
B. When checking the double check valve—either bench
tested or tested on the vehicle—two separately
controlled air supplies must be connected to the inlet
ports.
1. Install an accurate test gauge in the outlet port or
in a line from the outlet port.
2. Apply and release air pressure to one inlet port and
note that the gauge registers an application and
release.
3. Repeat by applying and releasing air to the other
inlet port.
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