HELP! Coolant leaking for head gasket when under pressure

[tonypark33]

I need help. I have a 2005 f250 6.0 it dose not use any coolant, no white smoke, no overheating, everything has been working fine. no problems, but today i flushed the coolant system and put in new ford premium gold coolant, did a pressure test at 16psi and it was fine for about 15min. then started leaking out of the rear of the engine on the passenger side. pretty sure its out of the head gasket. As soon as i bleed the pressure off of the coolant system the leak stops. and this is the only time i can get it to leak. has anyone ever heard of this happening? what should I do? please help!