Gotta say that those fittings are a pain in the butt. The nuts grip the tube so that you get a 1/8 of a turn in a tight spot, then the tube springs the nut back. I ended up gripping the tube with pliers but was scared to cause another leak.

 

In my case, it ended up being the center valve block. I found this out after doing the dryer fix then replacing the compressor then replacing the reservoir then replacing the rear block.

The test that eventually led me to this is the same as what you were describing. The pressure would climb while the compressor was on, but dropped once the compressor kicked off with no external leaks and no dropping over time.

 

This is encouraging. I had come to the same conclusion. I have the compressor kit incoming and I am sure i will find degraded desiccant and worn parts. But I believe the culprit is the reservoir control valve block. Also ordered the non-return valve. Fingers crossed but there will only 2 valve blocks remaining!

Thanks for posting!

Rob