I would think it would have to be sealed to draw the water back in or it just draws air from the out side.
I think that Ron is correct. The GM accessory unit either has parts/instructions to convert the radiator cap, or it provides an appropriate cap.
See figure 12. Up to 1954, track radiator caps were often 0 psi pressure (except for big trucks, in some models/years). So, there would be no need for a modification in a truck/radiator that has a zero pressure cap.
