gas line pressure test

I am finishing my basement and installing 3 new gas lines for range, gas fireplace and outdoor grill. I have all the lines run, but have not yet connected into the existing service at the house. I am pressure testing all the new work first and it does hold 15 psi for 15 minutes. It will even hold 15 psi for 3 hours, but when I check it the next day, the pressure is down to almost 0. To me, I would assume it would hold the pressure indefinitely. When I bubble test, I can't find anything.

Is this expected behavior, and thats why the inspectors will only look at the 15 minute test?

 

The pressure will vary greatly based on the temperature.

 

Is it steel pipe or csst? If it's steel (and not connected to anything) pump it up to 100 psi (or whatever you can manage) then spray. I don't know what the max would be for csst. Note that a plumbing supplier will carry bubble stuff that lasts longer than dish soap (doesn't evaporate as quickly).