I will look into the chemical test from NAPA. But why would the pressure seem to only build when the cooling system is passing through the heater core? Like I said last night, I bypassed the heater core by closing the valve, and the pressure seemed to disappear?

I also have another question. The cable on the inside that controls the temp is broke so I have to manually move the valve on the firewall, but how come when I move the temp from either hot or cold it still makes a difference even thought the cable is disconected?