So my set up is a 427W with a QF carburetor. The chassis has got about 25k miles on the clock. The engine is a Dart block from Fortes, and was installed about 5k miles ago. Recently (1000 miles or so ago) I've started to notice that the coolant does not draw back into the cooling system from the overflow tank. About 1000 miles ago I replaced the radiator (from FFR) as well as adding a solid lower radiator hose from Breeze. Those were the only changes.

My cooling set up is a stainless steel SPF surge tank with the radiator cap on it. The overflow tube comes out of the side of it and goes to the large Canton overflow tank mounted on the X brace.

I recently did about a 650 mile trip and noticed that the overflow tank ended up completely filling up and ended up overflowing by the end of the 3 day trip. The coolant temps were never an issue and the engine has never overheated - max temp on the gauge was 195* in 98* weather and stop and go traffic for 30 mins. Once the car is moving above 40 mph, it drops to 180 or so.

I'll try and explain my observations and what I've done. As best I can tell, the system appears to be staying pressurized, which would explain why the fluid is not being sucked back in when it cools down. So I've let it get fully warmed up, and sure enough fluid moves into the overflow tank. However once it cools down, the PVC hoses are still obviously pressurized. I can't squeeze them much at all.

Once after it was cooled down, I've removed the radiator cap, and about 1/2 a gallon of coolant comes gushing out (it's actually mostly water as i"m running 80% water with water wetter).

So after the next heat cycle was complete (filled it all back up etc) and the engine was cold, I opened the petcock at the top of the drivers side of the radiator, and got a gush of air then it stopped and no fluid came out. Once that stopped, I released the radiator cap, and then opened the petcock again, more air came out, and coolant drained from the surge tank into the radiator. Probably about 1/2 the volume of the surge tank drained into the radiator.

As best I can tell it appears that air is getting pumped into the system which would lead me to think of a blown head gasket. I'm using MLS steel head gaskets. However there are no other symptoms of a blown HG. No white smoke from the pipes, no overheating etc. Oil looks good, although since I'm using water, I'm not sure if it would be as obvious - I've had the chocolate milkshake experience before and would prefer not to repeat it!

So to check for a blown HG I rented a block tester kit, and the fluid never changed color to indicate the presence of hydrocarbons. The upper corrugated radiator hose got hot, so I'm pretty sure the thermostat was open.

Next I got a cooling system pressure testing kit - although in retrospect since the system was holding pressure, it seems obvious now that there would no leaks, and sure enough there weren't any. There have been no coolant leaks at any time.

So my conclusion (or question I guess) is could there be a leak in the head gasket that is causing air to get pumped into the cooling system without any other symptoms?

Any suggestions are appreciated!

Thanks