For a hydro test on a boiler, the pressure gauge must not show any drop in pressure for a set amount of time. No amount of pressure drop is acceptable. That is none negotiable. On the boilers I sometimes work with the smoke box doors are meant to be open so that any leaks between the fire tubes and tube plate are visible. If there is a very small leak, but no visible pressure drop on the gauge the inspector might let it go, but they also might not. It obviously also depends on why there is a leak. If its a pinhole or crack on a weld then obviously thats a major failure and will never be acceptable. If its a very small leak where a tube was expanded against a tubeplate, akin to small drops weeping through, then the inspector might let it go if the pressure on the pressure gauge does not drop at all. [pre][/pre]Same goes for leaks on mudhole gaskets etc. Inspector is final authority, if they are not willing to sign then you drain boiler, fix and try again.