BENDIX SD-01-700 User Manual

Powered by the vehicle engine, the 

builds the air pressure for the air brake system. The air 
compressor is typically cooled by the engine coolant system 
and lubricated by the engine oil supply.

The compressor's unloader mechanism and 

(along with a synchro valve for the Bendix

®

 DuraFlo 596

™

air compressor) control the brake system air pressure 
between a preset maximum and minimum pressure level 
by monitoring the pressure in the service (or “supply”) 
reservoir.  When the air pressure becomes greater than that 
of the preset “cut-out”, the governor controls the unloader 
mechanism of the compressor to stop the compressor from 
building air and also causes the air dryer to purge.  As the 
service reservoir air pressure drops to the “cut-in” setting 
of the governor, the governor returns the compressor back 
to building air and the air dryer to air drying mode.

As the atmospheric air is compressed, all the water vapor 
originally in the air is carried along into the air system, as 
well as a small amount of the lubricating oil as vapor.

The

 is the ratio of time the compressor 

spends building air to the total engine running time.  Air 
compressors are designed to build air (run “loaded”) up to 
25% of the time. Higher duty cycles cause conditions that 
affect air brake charging system performance which may 
require additional maintenance. Factors that add to the duty 
cycle are: air suspension, additional air accessories, use 
of an undersized compressor, frequent stops, excessive 
leakage from 

fi

 ttings, connections, lines, chambers or 

valves, etc.
The

 allows the air, water-vapor and 

oil-vapor mixture to cool between the compressor and air 
dryer.  The typical size of a vehicle's discharge line, (see 

column 2 of Table A on page 13) assumes a compressor 
with a normal (less than 25%) duty cycle, operating in 
a temperate climate.  See Bendix and/or other air dryer 
manufacturer guidelines as needed. 

When the 

 of the compressed air that enters 

the air dryer is within the normal range, the air dryer can 
remove most of the charging system oil.  If the temperature 
of the compressed air is above the normal range, oil as 
oil-vapor is able to pass through the air dryer and into the 
air system.  Larger diameter discharge lines and/or longer 
discharge line lengths can help reduce the temperature. 

The discharge line must maintain a 

 down 

from the compressor to the air dryer inlet 

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 tting to avoid low 

points where ice may form and block the 

fl

 ow.  If, instead, 

ice blockages occur at the air dryer inlet, insulation may 
be added here, or if the inlet 

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 tting is a typical 90 degree 

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 tting, it may be changed to a straight or 45 degree 

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 tting. 

For more information on how to help prevent discharge 
line freeze-ups, see Bendix Bulletins TCH-008-021 and 
TCH-008-022 (see pages 30-32).  Shorter discharge line 
lengths or insulation may be required in cold climates. 

The

contains a 

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 lter that collects oil droplets, and 

a desiccant bed that removes almost all of the remaining 
water vapor.  The compressed air is then passed to the air 
brake service (supply) reservoir.  The oil droplets and the 
water collected are automatically purged when the governor 
reaches its “cut-out” setting. 

For vehicles with accessories that are sensitive to small 
amounts of oil, we recommended installation of a Bendix

®

PuraGuard

®

 system 

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 lter, designed to minimize the amount 

of oil present.

™

®

supplies the 

compressed air for the braking system as well as other air 
accessories for the vehicle. The system usually consists 
of an air compressor, governor, discharge line, air dryer, 
and service reservoir.