Cab will not build air

Assuming that's attached to a leveling valve.... Check to see if there is air going into the leveling valve. Then see if any is coming out.

 

Yep, that's a leveling valve. When I bought my '06 KW T2000 a year and a half ago I had the same problem. WHAM! every time I went over a bump. I replaced the leveling valve, both cab air bags, and the shocks. I installed heavy duty shocks as I do a lot of driving on rough "roads". That took care of the problem.

You should also get a spray bottle, fill it with water, and add a splash of dishwashing detergent. Spray that on all your air lines and connectors after you have aired up the system and turn off the engine. Any leaks will start bubbling. Chasing air leaks and electrical gremlins is a constant battle on used trucks.

 

never mind

 

Don't bypass it. Not unless you want the rear end of your cab to sit a foot higher than the front of the cab.

Basically it sets the level of your cab air suspension. Its designed to compensate for weight (ie passengers, baggage/food in the sleeper etc).

 

You can't buypass it. It's there for a purpose. If it has gone to ####, then replace it. It MIGHT be that you can adjust the leveling arm. I tried that with my truck, but I couldn't adjust it enough to get the cab air bags to fill.

Bottom line is this: ALL components of a truck should be replaced on a set PM schedule. The leveling valve, shocks, and air bags in my truck were LONG overdue. This is likely your situation.

All PM schedules need to take into account the normal replacement of components. For example, what is the normal replacement schedule of your turbocharger?

 

That valve cost about $100 from a dealer.
Bypassing valve, will give you a rough ride. It might over inflate your air springs to fail.
Highly suggest you replace.
If you don't hear air leaking. You might just have a plugged line.
Disconnect the hose to the air springs and move your leveling arm to activate the valve.
If nothing happens check your feed hose. If no pressure there, follow the line back to its source.
Good luck.

 

That leveling valve does jyst what it says.
If you adjust it up or down it will affect the amount of air deployed into the bags (when operating correctly) and give you a softer or stiffer ride.

You'll actually see the cab rise a smidge.. or hear the hiss and it'll lower a 1/4 inch.

I doubt that is your problem here, though. If the system builds pressure and HOLDS but the bags are never inflating... it could be that valve is blocked.
If the system leaks down... you've got a bigger issue.