My 51 GMC has a L neck that runs into the back of the upper tank, unlike Chevys I think, that fill from the top. It's a "new" rig for me, so before when I took it out for a long hilly run yesterday I filled it to within an inch of the top of the fill neck. In theory the top tank should have still had a couple inches of air in it since the fill neck attaches near the bottom of the top tank. The temp guage seemed to stay on about 190 throughout the 40 mile trip, up hill or down, although I guess I wasn't watching it going downhill. From time to time I would stop, and notice a little fluid running out of the overflow tube. At the end of the run I could see the antifreeze level was near the bottom of the top tank and would guess I lost 1/2 gallon or so on the trip.
My maintenance manual mentions a "water level test cock", which I don't see, so I'm wondering if it just didn't push out all the fluid down to the proper level.
I'm running a 180 degree thermostat with a hole drilled in it, a newly recored radiator, it is apparently a pressure system as my overflow tube cannot overflow unless the cap is overpressured. I don't know what the cap is rated, but I think 8 lbs.
After this long rant, I guess my only question is how high should GMC radiators be filled?
