Building air problem

[JAsaro]

So I’m having a problem building air I replaced the governor and the air dryer the air dryer is a bendix AD-IS I know those are known for problems I sprayed the WD-40 into the unloader valve still no change I have to read the engine between 12 and 1500 RPMs in order to build air it’s fine rolling down the highway it’s just when I come to a stop there are no air leaks I’ve already checked I’ve been having this problem for a few weeks the only other thing I can think of is the air Compressor I could replace the head for $900 and change or replace the whole air compressor for 1100 and change that’s what I was quoted from my dealer
Cummins isx15
2250