Has the temperature changed?

Yes, there has was a variation of about 20oF.


Might the joints (if any) be creeping?

It's hard to say since most of the line is not exposed.


Have you calculated the change in volume that would be needed to account
for what you're seeing? Water's not terribly compressible.

I haven't, but this morning we refilled the line and purpuosefully
opened the faucet to experience a similar drop in preassure. The water
lost was about a third of a mug.

We are actually testing 3 lines in this fashion: 1 cooper line for
water and 2 copper lines for gas.

The good news is that the line for water has stabilized at 76 psi for
the last 72 hrs. so I feel pretty comfortable with this.

For the gas lines I'll start another thread.

Thanks!