It is possible that the initial loss of pressure is due to cooling of the air...if you are pressurizing with air. To get much of a pressure test.. I'd put at least 10-15 psi... wait a few minutes (maybe 5) then check pressure then check again in 15 minutes to see if it lost any pressure. I'm thinking that it should hold steady for 15 minutes... If pipes are accessible and exposed you can paint or spray on soapy water at joints and plugs.... good sudsing dishwashing liquid works great.