I was the OP in the Jan 26 thread "Any tips for finding air compressor
leak?". I'm here to report that I've had complete success ... YES!

One by one, I removed the potential leak-ers: the pressure guage, the
drain plumbing, the quick release outlet, the pressure switch, the
pressure relief valve and the Christmas tree (manifold) itself. As each
item was removed, I plugged it's port and checked for continued leakage.
If the leak had stopped, I would know that piece was the problem. I
also rigged pressure to that item and held it under water, looking for
bubbles. And with them all off, I checked the tank alone.

It was the last piece - the manifold itself that was leaking. Well not
itself exactly - it was the check valve built into it inside the tank.
I had soap bubble tested for this leak without finding it. Poor
technique, I guess. The valve was just a spring loaded disc against a
seat. The disc was phenolic like, so I tried making ones of nylon and
soft rubber. They both leaked worse than the original.

So I cut off the stub that the valve was built on and drilled and tapped
an NPT hole for a real check valve, which I got from McMaster-Carr. I
reinstalled everything, pumped it up, and 2 days later there has been
ZERO drop in pressure!

Thanks for all the responses. The posters who said "It's the check
valve" were right.

Bob