I've never heard of a "24-hour" test for propane systems. I'm only aware of the 15 minute test. How are you accounting for changes in ambient temperature? If you are using a 24 hour test, do you have information on what kind of pressure drop should be expected during this period? If a 5lb drop is nominal for the 15 minute test, and you were to extrapolate that, I would expect a very substantial pressure drop over 24 hours.
As for the solenoid not closing completely, you should replace that immediately. It is the key safety component of the system, and it needs to close completely.
I have always used teflon tape to seal the propane fittings with good repeatable results. Make sure the tape wraps in the opposite direction of the threads.