Some bad info in this thread. There are two gauge ports in the system, low side and high side. They should only be equal with the a/c off. If the system is properly charged, that reading should be close to ambient temperature; i.e. 80 degrees would be roughly 80 psi.
With the system in operation the pressures vary widely, high side could be up over 250 with low side down to 30. It varies depending on whether an orifice tube or expansion valve is used to cause the pressure drop.
There is a high pressure release valve either on the manifold block or the compressor itself, sounds like that's what's popping off.
I think the best course of action with this one would be evac the system, replace all o-rings and the dryer. Pull a good 28" of vacuum for at least 45 minutes. Inject some oil (depending on how much is recovered, 2-6 oz.) charge it with the specified amount on the data sticker. That should get it down to a nice 42 degrees outlet temp.