The only restriction to radiator cap pressure is what pressure your system will hold. Too much pressure you will spring a leak!

The point of a radiator cap to raise pressure is to raise the boiling point of the coolant. As the prssure rises so does the boiling point, in this way you get a few extra degrees of temp before boiling the coolant.

From wikipedia:

Quote:

So you can see low pressure equals low boiling point.