Volvo VNL slow building air pressure

I had this issue only on a Cat. It was some type of valve leaking on the back of the compressor. I couldn't hear it inside the cab but I opened the hood and could hear it hissing. Ended up draining all the air out and started it back up and it worked fine.

 

The main air line going back to the dryer assembly, some are steel braided and some have a fabric outer weave (steel braid inside) are usually mounted along the frame rail and routed back towards the drier assembly. Mine had a small hole in it as it passed over the rear engine mount that couldn't be seen, only felt (on bottom) and could only be heard if you really "keyed in" on it with the engine running (long to build air), of course with the engine shut down the air is depleted from the line and can't be heard. The dryer assembly has a check valve and will retain the air it has received. SOMEWHERE along this path there has to be a defect UNLESS the leak is elsewhere. You said it was a new compressor (new load/unload valves). Some air governors are mounted directly to the dryer and some remotely mounted at the firewall, in this case you also have air lines that have to be inspected in length for defects. The dryer may or may not be mounted against a small barrel canister (damper) that has o-rings between them. Maybe something as simple as your tank reservoirs pet ####s or tank drains (these should have been checked BEFORE all of those expenditures), just some ideas...maybe this helps.

 

It's a great chance that it's the steel braided signal line from the air compressor to the dryer, especially if it has never been replaced.

Even though it's steel braided on the outside, it's plastic tubing on the inside, which is susceptible to cracking when it gets old, especially after you loosened the connections up to replace the air compressor and dryer, just moving the braided line a few inches in any direction will crack the inner plastic tubing.

 

You probably have some serious air leaks or lots of small ones. All trucks with air brake systems will develop leaks at the air lines “ends” eventually. I find myself repairing all sorts of air leaks every few months.

I posted this before:

Searching for air leaks is a learning experience. Every time I do it, I usually discover more air lines! If you are attached to a trailer, set your trailer brakes (chock tires if have some), air up tanks to max psi (about 130), release the “tractor” air supply (very important), spray the “ends” of air lines, that is where I have found most of mine.

There are airlines above each air bag and brake chamber. Spray them and follow the air lines to the other end. All air lines should lead to your air tanks. Your truck may have 2 or 3 air tanks. Spray the air lines on the air tanks, including the release valve of each tank. There may be air lines under the cab above the drive shaft; under the hood by the steering wheel rod; and under the radiator. Also don’t forget to look for double female adapters that some mechanics may use to extend the air line length, and I usually forget to check the two air lines that go from my truck to the trailer.

 

easiest way I found to check a discharge air line for leaks is to adapt and back feed shop air from drier end of line into compressor with engine off. also make sure the turbo actuator which air kicks into around 90 (secondary) is no leaking from diaphragm on that ISX(it can be hard to hear but it can be felt. otherwise sounds like you got some good advice.

 

Try search block on the top right. We just beat this around pretty good.