BENDIX BW2789 Manuale Utente

The DRO valve can be installed in two confi gurations, 
series or parallel. 

Series Confi guration 
The series confi guration, shown in Figure 3, allows all the 
compressor discharge air to pass through the valve and 
into the dryer or reservoir. 

The parallel confi guration, shown in Figure 4, allows the 
DRO unloader valve to be mounted using a branch circuit.  
This branch circuit provides the fl exibility of installing the 
unloader valve in a more convenient location. The branch 
line should not run uphill or have kinks or sharp bends that 
may trap condensed liquids. 

1.  Locate a position in the discharge line of the 

compressor where the DRO valve can be mounted.  
The valve must be mounted at the same height or 
below the compressor outlet.

2.  Secure the DRO valve using 5/16” grade 5 or higher 

bolts.  It is not recommended to install the valve directly 
on to the engine or compressor.

3. For series installation, install a discharge line from the 

compressor to the DRO valve, as shown in Figure 3.  
For parallel installation add another discharge line 
into the existing compressor discharge line using a 
tee fi tting.  Connect the opposite end of this new line 
to the DRO unloader valve as shown in Figure 4.  The 
valve must be at the same height or lower than the 
discharge line. 

4.  For a series confi guration, the delivery line out of the 

DRO should be connected to the air brake dryer or 
reservoir tank.

5.  For a parallel confi guration, the delivery port out of the 

DRO must be plugged.

6.  The supply and delivery ports are 22 mm metric 

threads.  The control port is 12 mm metric threads. 
Consult a fi tting supplier for conversion fi ttings.

7.  Make sure there is a line from the governor unloader 

port to the DRO valve control port.

8.  Test all connections for leaks.

9.  Start the engine and allow the system to reach cut-

out pressure. The valve should open allowing air to 
exit the exhaust port.  A silencer, shown in Figure 2, is 
recommended to reduce the noise level out of the DRO 
valve when exhausting. Bendix provides a silencer that 
snaps onto the exhaust port the DRO.

10. Fan the brakes until system cut-in is reached. The 

DRO valve exhaust should shut allowing the air brake 
system pressure to increase until cut-out pressure is 
reached.

Note:  The operating range of the DRO unloader valve is 
from -40°F to 300°F.  The DRO valve must be installed with 
suffi cient discharge line length to prevent the air dryer inlet 
temperature from exceeding 160°F.

To prevent failure or freeze-ups, the control air into the 
DRO valve must be free of liquid water and fl uids and the 
maximum inlet pressure to the DRO valve is 174 PSI. 

TO THE 

CONTROL 

PORT OF THE 

AIR DRYER

TO THE SUPPLY 

PORT OF THE 

AIR DRYER

DRO 

UNLOADER 

VALVE

FROM THE 

RESERVOIR

GOVERNOR

COMPRESSOR

TO THE 

CONTROL 

PORT OF THE 

AIR DRYER

TO THE SUPPLY 

PORT OF THE AIR 

DRYER

DRO 

UNLOADER 

VALVE

FROM THE 
RESERVOIR

GOVERNOR

PORT 21 MUST 
BE PLUGGED