GMC 2003 C4C042, Owner'S Manual, Page: 300 / 386

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Air Brake Systems (If Equipped)
If you have air brakes, it is important to get rid of
moisture in the system. Moisture will damage your
system if it isn’t removed daily.
There are two ways to do this. One is automatic if your
air
-
brake vehicle has the optional moisture ejector.
If your vehicle doesn’t have that option, however, you
must drain the air reservoirs occasionally to be sure the
air dryer is working properly.
Drain the air brakes at full system pressure. To be sure
you’re at full pressure, check your air pressure gage.
It should read at least 100 psi (692 kPa).
Moisture Ejector (Option)
If you have air brakes with a moisture ejector, the valve
automatically ejects moisture from the reserve tank
when the air compressor cycles.
Air Dryer
If your vehicle has air brakes, you have an air dryer.
This collects and removes dirt, moisture or other foreign
matter from the air prior to entering the brake system.
It is mounted on the passenger’s side frame rail.
If your dryer is a Bendix
-
Westinghouse, it has a
filter that you need to change at intervals. See your
Maintenance Schedule for more about servicing
this filter.