DD15 constantly building air and purging every 20-30 seconds.

[DVT873]

"is constantly purging air going down the road despite no visible drops to 100 psi on the gauge to even be running the compressor."..............................................................

This is one of my pet peeves ever since dual circuit air brake systems became the law. So first off do not think in terms of air tanks. Instead think of reservoirs. A tank can and is often subdivided into more than one reservoir. On some heavy duty trucks there is one tank subdivided into 3 reservoirs.

Air leaves the compressor and goes to the air dryer if the truck has one. Today virtually every truck has an air dryer. After the air dryer it goes to the supply reservoir aka "wet tank". Air leaves the wet tank through two lines going out through one way air check valves. The air splits with one line going to a "primary tank" and one going to the "secondary tank" Your dash air gauges are hooked to those two tanks.

There IS NO GAUGE in the supply "wet tank" part of the system. There are plenty of leak points there including the air dryer. The air compressor governor line is connected in that part of the system. Usually to the the reservoir tank. Any leaks in that part of the system will make the compressor run or "cycle"

Air loss in this part of the system WILL NOT show up on your dash gauges. But you compressor will be running. Every time the compressor cuts out the air dryer will purge. That you usually hear. Ever hear a truck idling and every few minutes you hear air going "fsssstt" and stopping? It cycling because of an air leak. What you hear is the air dryer purging. If I had a truck of my own I would put in a gauge so I could see the supply side of the system.

Many times the problem is in the air dryer. These can be a PIA to find if the air leaks off quick. All the NAV's will have a male Compucheck fitting in one of the tanks. The reservoir it is in is always the supply or "wet tank" reservoir. It is there so they can be aired quickly at the end of the assy line for the brake check tests.

Hook shop air up there to air up w/o the engine. Also handy if you need to air up to release and cage brakes when the engine won't start.

Quick link to a generic air system set up.

http://3.bp.blogspot.com/-_NLyluAuvww/VT...system.gif

There are better pics out there but this gives an idea. The check valves aren't blindingly obvious but they are there.