Squeeze: This is one of those GMC thangs.
The pressure relief valve in the radiator top tank is standard on all '47-'55 GMCs. The GMC radiator, unlike the Chevrolet truck radiators are not only different but use a sealed cap, not a pressure relief cap. There is no overflow on the orginal radiator cap neck. Instead, GMC used a little "button" style pressure relief valve located at the exact expansion level in the top tank to allow excess coolant out and create the necessary head space for proper pressure (4 psi) operation.
The valves are indeed available at any radiator shop that deals with truck radiators as they are still used in some truck radiators today. I do not know the part number but will try to grub it up and will post it here if I can find it. It's a common part at a good truck radiator place so DO NOT throw the old one out.
By adding an expansion tank you are in effect running your system at zero pressure (unless you pressurize the expansion tank) and allowing the coolant to simply ebb and flow with pressure.
With the proper reflief valve, these old radiators work fine using the top tank as the expansion tank.
