Coolant Leak

[bizzle]

For the past few months I've had a coolant leak that I can't figure out. The system holds pressure overnight, every time I check. I pressure tested the system and it lost 2 psi in the first few minutes and didn't drop any further. My father said it could be leaking from the diaphragm in the pump and when I am checking the coolant (which I've been doing every few days), that is relieving the pressure in the system which is causing the bad diaphragm to leak until there is pressure in the system again, because he says they normally won't leak when under pressure. Anyone have any ideas? The water pump is a mother fucker to get to so I don't want to replace it if I don't have to. The leak isn't massive and I've never noticed any puddles but I keep finding coolant in the engine bay.