Question: I use my “shop air” compressor to put about 80 psi of air into the system after I removed the refirgant. The pressure holds and does not show a loss of air on the gauges.
when I do a vacuum in the system, then check for leaks, my “gauges” show a slow leak after 5 minutes. How can the air show no leak when I apply air, but I have a leak when I do a vacuum. Aren’t both test valid to verify my AC system will hold refrigant? I replaced the compressor and accumulator.

 

this can happen...pressure moves a seal one way...vacuum the other.
add uv dye...charge it...run the system...leak check with a uv light and electronic detector. if no hits...run it. check in a week.

 

I wouldn't use shop air for testing pressure integrity of a system, as the shop air can contain some water. And water gets absorbed by the R134a refrigeration oil and the desiccant in the drier/accumulator. Shops either use dry nitrogen or a single can of R134a for such testing.

So now you're in a pickle !

 

He's gonna have to install a new accumulator, ATER he gets all the air out with a vac pump. Had my low-side intake valve leak on my 23-year-old Chevy pickup. I suppose that's good as it only added dye and an ounce of PAG after all those years. Condenser probably needed to be replaced anyhow. Compressor still running good. Got it back down to 38F but since its cold I'll wait and see what happens when it warms up some.