Tom,
Do you have a shop manual for your engine and/or truck? The GMC radiator and engines are distinctly different from those used in same year Chevrolet trucks.
I think that a 1952 Chevrolet small-truck radiator cap was 0 lbs pressure. In 1954 that changed to 4 lbs pressure in small trucks and 7lbs pressure in automobiles.
If you have a GMC radiator, I think there is a pressure-relief valve on the radiator. Maybe a GMC person can provide information about using a pressure cap on these radiators (maybe someone above was specifically referring to a GMC radiator/engine?).
Tim
