Just replaced radiator with a ron davis 2 core 1" aluminum and a milodon high flow pump with a 180 thermostat. could not find a robert shaw high flow piece so bought a hi-po stant. If you rev the engine up you can watch the upper hose trying to collapse. I havent checked the coolant level yet, hasn't cooled down enough to see how low or why ! Any ideas ?

The upper hose is the outlet, right? My guess is that you still have some air in the system after the component change. When you rev it up, the pump may be forcing some coolant into the air spaces and pulling a partial vacuum against the still closed thermostat.

I would let it idle and warm up enough to let the thermostat pop open at least once, you will proobably see a big bubble of air burp out and then have room for more coolant.

The upper hose is the return hose. Normal water pumps draw coolant from the lower hose and the coolant returns to the radiator via the upper hose. I can't imagine why you would be getting vacuum pressure in the upper hose. I've never seen a thermostat that seals so tight you could pull a vacuum against it anyway. If anything, your cooling system should have positive pressure not negative pressure as thing heat up.

If you end up pulling the thermostat again for any reason, drill a 1/8 hole it the base to allow the system to burp any air that may be trapped when refilling the system.

You aren't using a serpentine or reverse flow water pump are you? I have no idea what would happen if you did, but I'm sure it wouldn't be efficient.

I have seen this happen before and Corey is absolutely right!

What Corey left out is that air is in block and the pump is working to shove water against that airpocket essentially compressing the airpocket and with the thermostat closed and the probability that the top hose is not full either, this sucks the hose together when reved because it is drawing against a fixed volume in the upper hose!

You can jack the front of the car up and burb all the air out when running and fill to the top and only close the system when it's completely topped off and cold! This works well as well as adding surfactants such as WaterWetter to help eliminate trapped air.

I also suggest pressure testing the Rad Cap as the system warms up it should build pressure and that pressure will keep the hose from collapsing.

Run it thru a couple of hot/ cold cycles. Leave the cap on til its completly cool (overnight if possible) pop the cap and refill. If you have an overflow bottle and a good cap the coolant level will be at the top of the radiator. With no overflow bottle the level should be about an inch and a half down when all is said and done.

SORRY I HAVENT THANKED FOR THE REPLIES, BEEN BUSY TRYING TO GET READY FOR CRUSIN THE COAST IN MISSISSIPPI. I LET THE CAR SIT OVERNITE THEN TOPPED OFF THE RADIATOR AGAIN, END OF PROBLEM. IF ANY OF YOU EVER GET A CHANCE TO GO TO THAT EVENT BY ALL MEANS DUE SO. JUST MAKE SURE YOUR COOLANT SYSTEM IS UP TO SNUFF OR YOU'LL DAMNN SURE RUN HOT.