You might have something like a crack in a head or cylinder wall which opens up with heat, and the pressure you can bear on the rad will not show that
Rig yourself a spark plug air supply, take the check valve out of your compression tester, etc, and air up one cylinder at a time and see if you get bubbles back into the rad.
If you pull the valve gear loose, you can air them up at bottom dead center, with the valves closed. That way the swept area of the cylinder wall will be exposed.
