Leaky system, propane, sounds like a recipe for success! A flick of the Bic (R) will help you find the leak.

Seriously, it will be easier to find the leak with a refrigerant in the system and an electronic leak detector. A shop can probably do this for a minimal fee.

And no, the ability to hold a vacuum does not mean that the system is tight. The converse is true though; if you don't hold a vacuum, you will never hold pressure.