I'm not a Tech, but read this several hours ago, and no one has answered yet.
As the coolant system heats up, coolant level should never drop in your expansion tank, as long as your coolant system is "gas free". With a "gas free" system, as the coolant heats up and expands, there is no where for the expanding coolant to go but your expansion tank, in which level should rise about one inch, and develope normal system pressure. If you have gas in your system, the gas bubbles collapes under pressure and expansion tank won't rise as expected, and system pressure will not build as required. As the heated coolant system cools after shutdown, the contracting fluid decreases system pressure, and could go to a vacuum. The expansion tank cap has both a pressure relief, which lifts if pressure increases about 15 PSI. This same cap also has a vacuum relief, which is necessary to break vacuum developed when the system cools, and this is suppoosed to allow any gas in system to go back to expansion tank, which allows coolant to replace this vent gas.
If you are only getting bubbles when cap is removed, I'd suspect the vacuum relief in your expansion tank may be faulty. If you remove the cap, wash it, you should be able to suck air through the cap. It takes a slight amount of vacuum before you'll suck air through it. If you can't suck air through it, I'd replace the cap, which cost about $8. Now watch your level over the next several cool-downs, and fill as necessary. Some that had gas in system needed to add 1-2 qts to system until de-gassed. The system should de-gas itself when cooling down with a working cap.
Does the upper radiator hose get hard when coolant temp reaches 190 Deg F? If not, you probably have gas in your coolant system. Another sign of gas in system is if your upper radiator hose collaspes after cooldown. There has been great discussion in the LLY section the past few months, so you could do a search to read more.
What's the proper expansion tank level? Most say the seam 1/2 way up tank, where the "Full Cold" arrow is. Others say to fill when cold, to the plastic tab about 1 1/2 inches above the mid-way seam. I ended up filling to the tab above the "Full Cold" words. With less gas in tank above fluid, system pressure should build quicker as fluid expands during heat-up, toward 15 PSI. I have yet to blow out any coolant, with my level here, so I don't think it's too high.
If you have continuous bubbles in your expansion tank with motor running, most seem to think the likely cause is a bad head gasket. I'm not a Mechanic, so if you have this issue, be sure to confirm it with a qualified Mechanic. I wouldn't want to mis-lead you.
Again, I wish qualified others would have answered, but none have yet. These are a couple of sugjestions till you here from others.
