I have the GMC radiator, just like the one in the manual. It has the 4 lb relief valve over to the side. The main filler does not have a spill pipe/hole in the neck so no matter what cap you put on, there will be no regulated relief through the main cap. The pressure relief comes from the spring loaded seal at the bottom of the cap assembly and when that opens, there is supposed to be an exit hole above that and below the top of the cap. This hole is missing in the GMC type setup, at least in the manual and on my radiator.
