Michael ,
The original 400 - 403 radiators were pressurized to about 2psi .
There is a ball and spring incorporated in the outlet of the overflow pipe .
I have recently fitted a new stronger stainless steel spring as a trial to my two 400s. My radiator pressure tester shows that they now run at approx. 8 - 9 psi .
At that pressure I have not encountered any problems, like leaks in the system .
So if your water pump, radiator hoses and other features of the cooling system are in good condition, I would not expect you to have issues with a 7 psi cap.
The alloy radiators I have heard are more efficient than the original brass type but they are known not lasts as long as the original type.
GeoffD
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