Cooling System Pressure Problems

All pertains to a 1985 Jeep CJ7 2.5L 4clylinder:

About a month ago 2 leaks broke out in my cooling system. One around the water
pump the other was a freezer plug. I replaced the water pump and replaced the leaking
freezer plug(which literally had a small whole blown through it) along with 2 other
freezer plugs near by. I then put the system back together and have been having
problems ever since.

I bled the cooling system to remove the air and now new leaks just keep springing up.
First the thermostat housing, which I replaced along with the thermostat. After this
leak I ended up getting a radiator cap pressure tester which I put on the system. I
pump it up to 16psi, the radiator cap is a 16 pound one, and no leaks. I then run the
car for 10 minutes and the cap pops up under the pressure allowing fluid to volcano
out, instead of releasing water through the overflow tube. I believe this is due to way
too much pressure. I then put the radiator cap pressure tester on the car and let the
car run. The system built up to 20psi in about 5 mins and I shut down the car and
released the pressure. Didn't want any hoses popping or anything else in the system.

I though maybe the new thermostat wasn't opening causing the pressure to build up,
so I pulled it out and tested it. The thermostat worked fine. I ran water through the
radiator, thinking it might be clogged, water runs through fine. So all the parts of the
system seem to be working.

I then suspected a problem with the head gasket so I preformed a compression test on
each of the cylinders. Each cylinder built up to 140psi after the engine turned over 4
times. No liquid was evident in any of the spark plugs. It doesn't seem like any liquid
is in the oil or oil is in the coolant fluid.

So I'm running out of ideas any other ways the pressure could be getting this high?