I was getting rid of the heater bypass valve this weekend and ran into something I am not sure of. I added coolant to the radiator and started the car to bring it up to normal temp with the radiator cap off. I could see coolant circulating and saw the coolant level drop in the radiator so I added some more to bring it up to normal level. After running a while the coolant level in the radiator would slowly drop and then slowly rise and overflow the top of the radiator for a second then drop back down. This happened repeatedly once the temp got around 180 degrees. It acts like there is an air pocket in the system or something. Does anybody have any ideas?

 

Probably opening up the thermostat and it's trying to burp the air pockets. I usually just compress the radiator upper hose with the motor off till it burps all the air out the radiator that way, continuely fill it up with antifreeze as it drops. Then let the motor start and warm up and constantly keep topping the tank. But the way you're doing is more practical if you don't mind making a mess eventually.