somewhere i saw a picture of one apart. all it basically is is a cartridge type valve that is held in with a snap ring. they were set for 4 lbs. so get a higher setting cap and run it and see if it works if it does then everythings good. i add a clear hose to the end of my overflows just so they stick out far enough from the bottom of the radiator that i can see the end of it and use it as a telltale as to when the caps or buttons are pushing out fluid.
you should have a thermostat. if you dont then it was removed for some reason. if you add one be prepared for something out of the ordinary. hi collant temp, leaking relief button, blown ranco valve but then i dont know why you would need one at your location. every truck i had that i addded a thermostat to something happened and then i say thats why they took it out. i fix the reason then reinstall and life is good again for a while.
