BENDIX TCH-008-041 Manuale Utente

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© 2006 Bendix Commercial Vehicle Systems LLC All rights reserved.  6/2006 Printed in U.S.A.

TCH-008-041

6-20-2006

N/A

3

The Bendix

®

 AD-9

™

 air dryers are available with various purge valve confi gurations; hard seat, soft seat 

and DLU discharge line unloader types.  The difference between these types relates to the type of sealing 
surface for the turbo cut-off valve.  All AD-9

™

 air dryers contain two valve sections and have a rubber (soft) 

seat for the actual purge valve, but the turbo cut-off valve section can be one of three types:

1.  Hard seat purge valve has a metal to metal turbo cut-off valve sealing surface.  Most AD-9

™

 air dryers 

come standard with the hard seat purge valve.  

2.  Soft seat purge valve has a bonded rubber ring on the piston that contacts a metal seat on the housing 

to make the turbo cut-off valve seal.  This seal is extremely air tight for applications that require zero 
leakage, such as the Bendix

®

 EverFlow

™

 system and drop-in style air dryers which use older Holset 

type E compressors.  

3.  Discharge line unloading (DLU) purge valve has the turbo cut-off valve bypassed by removing the 

middle o-ring sealing surface from the body.  This valve is used to positively vent the compressor 
discharge line, such as when the vehicle has a non-unloading compressor.  

NOTE:  Although a soft seat purge valve can be substituted for a hard seat purge valve, a hard seat purge 
valve should not be substituted for a soft seat purge valve.  DLU purge valves cannot be substituted for any 
other type.

Figure 1 - Identifi cation of Hard and Soft Seat Purge Valves

Hard Seat

Soft Seat

Metal 

Black 

Rubber