i did headgaskets on my equinox 3 yrs ago. i notice the surge tank level is about 1" below cold fill line. the tank is pressurized and has a screw cap. if i unscrew cap i almost always hear a pressure release and the level in tank rises to the full line. this is on a cold motor. 24 hrs after shutdown. where is the fluid coming from? from expanded rubber hoses? rad hoses? heater core hoses? all rubber hoses? you cant compress fluid.
Yes I know fluid expands as it gets hot.
With cap off level is correct. I drive car and the level is 1”below full when hot. So. Level is dropping when hot. I let motor cool. Level is still 1” below full. Remove cap, level rises.
You are putting too much into the expansion of liquids as a factor… Water: will only expand a few percent between 32-degrees (just above freezing) and 212 (just below boiling). Otherwise many pots put on the stove would be spilling over on the stove… Ask your Mom… L 
L . . .
It is more likely that when you run the engine the hot coolant softens the rubber hoses and they stretch a bit, holding more coolant, and when you release the cap, the rubber hoses relax returning to a smaller diameter pushing the coolant into the over-surge tank. If this is happening, then you might want to have your rubber hoses checked, they should not be doing this to the extent you described.
But in a vehicle the cooling system is under up to about 16 psi, and runs in the 220-240 degree range normally, so it will expand even more without boiling over…
Unless I am missing something here, you should not have pressure on the cooling system on a cold engine/coolant…
Matter a fact, for safety, you tell the new guys in training to never open a system if you can’t squeeze the upper hose together and feel your fingers touch…
I would hook up a coolant pressure tester to the system and with zero pressure on the system start the vehicle, and one see how fast it builds pressure, but (if safe) let the vehicle go through a couple of fan cycles while watching the pressure, then let it cool down overnight and see what/if the pressure gauge is showing before opening the system back up…
I am wondering if you don’t just have a bad rad cap…
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Ok, but why does level in surge tank drop as motor heats up?
Isn’t that the purpose of a surge tank? Versus a reservoir?
That’s a good question. Neither of my cars uses the surge tank method. And I’ve never experienced the coolant level to drop as it gets hotter. It has always gotten higher, never lower. Either in the radiator or in the capture bottle. If all of the coolant in your case seems to remain in the cooling system, meaning there is no escape path, there must be a place somewhere in the cooling system coolant is going when the coolant is hot that it can’t when the coolant is cold. Where that is, no idea. Knucklehead diy’er opinion is all. But it is an interesting question.
I did head gaskets on this rig 3 yrs ago. And I did the same on a similar rig last month and drove it for several weeks to monitor cooling system, the level would rise slightly when warm. And next day when I removed cap there was no pressure in tank.
I’m chasing a cold lifter tap and might replace lifters in the near future.
There might be some air in the system, did you open the bleed screw once again after several miles of driving?
Does the surge tank heat up? The air in the tank would then push some coolant into the system.
I like the ideas in your original post. Might even be the resivour tank and hose. It holds pressure so no immediate worries. Radiator cap was suggested earlier, assuming it is a resivour cap, could give replacement a try.
I am thinking more and more that this is acting like a messed up radiator cap, as I mentioned in post 3…
Cheap enough, try it, or heck, swap rad caps from one of your other rigs, I think you have more than one… lol
EDIT: if the system is properly bleed, which since the OP has done multiple head gaskets on these vehicles, I am assuming he does know how…
If the air in the surge tank heats up it would expand. However if that expanded air (thus pressurizing the tank) can push some coolant into the system then the system (engine and heater core…) must have air in them… But isn’t really a “push against a shove?” As the coolant and air in the system would also be equally pressurized?
So, I go back to my old premise… The OP wrote, “if i unscrew cap i almost always hear a pressure release and the level in tank rises to the full line.”
If the OP unscrews the cap and releases the pressure, what is pushing the coolant into the tank. If there is not a lot of air in the system, then it must be the squishy, squashy hoses that expand under heat and pressure.
And if so, the OP needs to replace those hoses…
The pressure expands the hoses and plastic tank making more room for coolant. Let the pressure off and there is less room for coolant and the level rises. Could happen.