C15 very slow to build air pressure

I have an 03 C15 in a 379. All of a sudden it takes upwards of 4 minutes at 1000rpm to fill the system from 0 psi. I've checked the pressure out of the compressor with a compression tester at about 150psi at 900rpm. I've checked the braided line to the dryer for air leaks using shop air. And I replaced the air dryer and no change. Any ideas? Thanks.

 

In the compressor head?

 

Very quickly.

 

Yeah, I was pretty much down to that but I couldn't make sense of why it showed 150psi compression. But if it is a valve it would still build pressure from 1 cylinder, just slower. Makes sense. Any idea where to find specs on rebuilding it?