Air pressure not building.

You either have major air leaks, drier issues, of compressor issues.

You need to plumb in shop air and see if it builds, and see where air leaks are.

You can pull the discharge line off the compressor and see what the pressure is, or see if you can hold a gloved finger over the discharge fitting

 

Compressor may be worn out. Take braided airline off of the back of the Compressor. Put you thumb over the the outlet on the back of the Compressor, it you can hold your thumb on the outlet, your Compressor is worn out

 

The only two things I’ve had do something similar to that is a bad air leak or the air governor on the compressor going out. Those governors aren’t hard to replace, but they can be a real pain to get the air pressure adjusted right in my experience.

 

Stuck one way valve or maybe air dryer leaking. But of course there's 1 million different things that it could be. Pull Your trailer parking brake see if that changes anything tractor parking break,See if that changes anything. And of course, it could be the compressor or the Air governor.

 

I'll go with air leak for $500 Alex.

 

Compressor Unloader valve.