[Rick Parker]

There are 2 main types of radiator caps and it's possible to have the incorrect one installed. If you have a coolant recovery or over flow tank mounted below the upper level of the cooling sytem the cap will require a seal on the inner part of the cap as well as the outer part of the radiator cap to maintain a seal on the top of the neck so when the system cools it will be able to maintain a vacume and allow coolant from the overflow tank to be pulled back into the radiator.
If you have the tank, and the cap being used only has the seal "inside" the neck, it will maintain pressure in the system until the rated pressure is exceeded and it will allow coolant to be purged but not to be drawn back in. If you do not have an overflow tank the cap used will not require the outer seal on the cap. This is what was used on all cars in the 60's, because very few had a recovery tank.