A radiator specialist should be able to pressure test the system for a start. Secondly the CPC catalogue recommends a 15lb cap as they supplied to GMH originally. It should be a coolant recovery cap. You can clean around and underneath the small circular valve plate. The slightest dirt can cause it to lose pressure easily which then reduces the boiling point. Keep an eye on the recovery bottle to ensure it is working properly and should be reasonably clean water.