411 S5 correct radiator cap pressure
 

What would be the correct relief pressure for the radiator cap on the expansion tank?

The unvented cap on the radiator itself is, most probably, the same as used on the MG Twin Cam.

Regards

Thomas

Just learned that the radiator cap on the expansion tank has to be the 15psi type.

Thomas as best I understand the cap on the radiator should be blanking cap only and the cap on the expansion tank should be around 15 lb. That is what I have on my car and it works well.