There are only two things that cause pressure in the cooling system - expansion and cylinder compression - the water pump is only an impeller to move the coolant - if the radiator is clogged, the coolant will not move and the engine will overheat - the water pump is only an impeller - unlike an oil pump with tight tolerances, the impeller does not generate high pressure - 4 to 8 psi is about it and the radiator pressure cap is about 8 psi, so the water pump could not create enough pressure to push past the cap.
There is a remote possibility that crankcase pressure could pressurize the counter balance chamber and push past the water pump seal to pressurize the cooling system, but if it were doing that, there should be iol in the coolant
See if you can get someone to pressure test the crankcase - should hold 7 lbs and no more than 15 - above 15 psi, the seals can be pushed out of engine case
I believe the problem will come down to a compression leak into the cooling system
Shop Owner and Mechanic with over 50 years experience