Bendix Commercial Vehicle Systems DC-4 User Manual
Bendix® Double Check Valves
SUPPLY
SD-03-2202
DELIVERY
SUPPLY
SUPPLY
SUPPLY
DELIVERY
SHUTTLE
VALVE
O-RING
CAP
BODY
FIGURE 1 - DC-4™ DOUBLE CHECK VA LVE (SHUTTLE TYPE)
SHUTTLE
GUIDE
DESCRIPTION
Double check valves are 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. They may be
used for directing air flow for specific functions or to select
the higher pressure of either of two sources of air as a
supply source.
Bendix manufactures two types of double check valves:
shuttle and disc. Although the valves are somewhat dif ferent
physically, the same function is performed by both types.
The difference in the design of the two valves is that the
shuttle type has a movable shuttle to seal off the lower
pressure source, whereas the disc type has a movable disc.
DELIVERY
O-RING
CAP
VALVE
GUIDE
FIGURE 2 - DOUBLE CHECK VALVE (DISC TYPE)
VALVE
DISC
BODY
OPERATION
As air under pressure enters either end of the double check
valve (inlet port) the moving shuttle or disc responds to the
pressure and seals the opposite port, assuming it is at a
lower pressure level than the other. The air flow continues
out the delivery port of the double check valve. The position
of the shuttle or disc will reverse if the pressure levels are
reversed. Double check valves are designed so that the
shuttle or disc can never impede the backflow of air in the
exhaust mode.
Figure 3 (see page 2) illustrates a typical use of a double
check valve to control a given device, such as trailer brakes,
from either of two control sources.
Figure 4 (see page 2) illustrates a typical use of a double
check valve 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. It is therefore suggested that performance of
the double check valve will be optimized if it is mounted in
the horizontal position.
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Reservoir
Brake Valve
To Trailer
Brakes
Trailer Hand
Control Valve
Double
Check
Valve
FIGURE 3 - DOUBLE CHECK VALVE: CONTROL OF
SYSTEM FROM EITHER OF TWO CONTROL SOURCES
™
PP-7
Valve
™
PP-1
Valve
Trailer Supply
Spring Brake Control
Double Check
Reservoir
Valve
Reservoir
FIGURE 4 - DOUBLE CHECK VALVE: SYSTEM WITH TWO
SUPPLY SOURCES
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 DC-4
valve should be inspected every 6 months or 1500 operating
hours, whichever comes first, 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 foot brake valve and note that the
brakes apply and release on both tractor and trailer.
2. Apply and release the trailer control valve and note
that only the trailer brakes apply and release. With
trailer control valve applied check exhaust port of
foot brake valve for leakage with soap solution.
Permissible leakage is a 1” bubble in 5 seconds (100
sccm).
3. Apply and hold a full foot brake valve application.
Check exhaust port of trailer control valve for leakage
with 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 which case it may
be necessary to disconnect this line to make leakage
check.)
B. If the double check valve is to be 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 outlet port.
2. Apply and release air to one inlet port and note that
gauge registers application and release.
3. Repeat by applying and releasing air to other inlet
port.
4. Leakage check should be performed at inlet ports of
valve in the following manner:
a. Disconnect the line from one inlet port.
b. Apply air to other inlet port and coat opposite
inlet port with soap solution. Permissible leakage
is a 1” bubble in 5 seconds (100 sccm).
c. Repeat Step “b” applying air to other inlet port
while checking opposite inlet port for leakage.
If the double check valve does not function as described or if
leakage is excessive, it is recommended that the valve be
repaired or replaced with genuine Bendix parts. The following
instructions should prove helpful:
DISASSEMBLY
1. Remove the end cap(s) from the valve.
™
2. Remove the grommets (if applicable).
3. Remove the shuttle and/or shuttle guide, disc and/or
disc guide (depending upon type of valve).
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