[rotorwrench]

The bung isn't designed for a pressure cap. Most of the early Fords up until around 1941 just have a rubber washer to seal the cap. This is why Skip Haney started making his 3 PSI check valve for installation in the overflow line. It allows some pressure to build to increase the boiling point without damage to the radiator if it is in good condition. As the metal tubes & tanks get thin with corrosion and repeated repairs, they tend to fatigue to the point that they will no longer hold much pressure. The cap seal has to be able to hold 3-PSI as well or even the pressure valve won't work.