[dsraven]

I would loosen the master from the booster and check for dampness in that cavity. grab some brake clean and dry the area up. then tighten it back up and pump the pedal a few times like you were. then go back under and loosen/recheck. if it is dry then do the baby powder trick and leave it for a few days, pumping the brakes like you have been doing. if it is a lid seal leak that will show up easily. sometimes the lid seals on rebuilt units aren't that awesome and the little fountain of fluid that happens when when the brakes are released can sometimes cause enough disruption in the reservoir so the fluid makes it's way out the lid seal. the lid should also be vented so you could check that too.