That is why you "crack" it open.

 

wbama and hotel are both right. Let engine cool to touch. Drain radiator fluid out. Remove bleed plug, fill up radiator and overflow tank. Surround bleed plug area with rags to catch coolant. Start car and let warm up as fluid heats up it will expand and start to push fluid out the bleed plug you can now stop the car and put bleed screw back on. Then I like to do the same with the radiator cap, remove, let car warm up when level starts to overflow, put radiator cap back on. Then get in the car, when it is warmed up fully start the heater and run it for a minute. Let the engine cool back down again so you can then top of the fluid levels in both the radiator and overflow. That is how I do it with no air.

 

One thing i would look at is the cap, if it looks like it is in rough shape it can be leaking and causing pressure to change. its a 10$ fix. I had a problem like this on my 95 Toyota, and changed the thermostat thinking that was the problem, but turned out it was the stupid cap...