If you remove the radiator cap while the engine is running, if you see bubbles, it is a blown head gasket. The thermostat needs to either be removed or be fully open. It is best to do this with the thermostat removed and the radiator topped off. Just run at slightly more than an idle and hold it there. You must be able to see the coolant in the radiator for this test. Pressurizing the system isn't going to do a thing, because you can't see an internal head gasket leak. All that is going to do is push the liquid into the cylinder and ruin your engine for certain. The combustion chamber has more pressure than the radiator, so it pushes the water out. If the radiator is pressurized and the engine isn't running, the coolant will be pushed into the cylinder. True, the pressure gauge on the radiator pump will continue to drop, but that could be as the result of a bad hose clamp or any other radiator related problem.