2000 Pontiac Grand Am loss of all coolant all at once

To bleed the system I started the car with the radiator cap off.(There is a bleed screw on it but last time I tried using it it snapped off because its made of soft cheap brass and I ended up having to replace the whole pipe it was attached too) so anyway with the cap off I revved the engine up to 1500 rpm's for three 15 second intervals. Then replaced the cap turned the heat up to high and let it run at normal operating temp. This did nothing to help and the heat was cold during this process. Thanks again for any input

Oh and sorry its a 3.4 liter v6

Had a dye test done on the coolant. No exhaust gas in it and no coolant in the oil or oil in the coolant or white smoke I think its pretty conclusive that its not a head gasket. Also conclusively not an air pocket. Any other ideas. Clog in the heater core. Coolant temperature sensor. Etc.

Thanks I think your right about the air pocket. My only other question is what could be causing it to keep getting air pockets. Again its not the head gasket and i'll seem to work an air pocket out and the car will run fine for a couple days then out of nowhere dump all the coolant out of the overflow hose all at once again. Pressure tested and was fine. Everytime I bring it to the mechanic it doesn't happen and they say nothing is wrong. Also would doing a coolant flush help?

The heater works sometimes. It will be cold then the temp gauge will go up then back down and after that the heat comes on. Unless the temp goes all the way up and dumps the coolant out of the hose

Yeah. Did what you said. There is definately air in the system my only other question is what could be causing air to keep getting back in the system after its bleed.I'll bleed the system and it will run fine for one start up.I mean I can drive it 100 miles or more. Then once I shut the car off and start it again a couple hours later the heat doesn't work temp fluctuates and the temp either goes up three quarters of the way and back down to normal or goes all the way up and all of the coolant dumps out of the overflow hose onto the ground.I refill the tank after that and it runs ok for the rest of the trip. No matter how far. Then the next day I start it again. Same problem as before. Have had three mechanics look at it. Including the dealership. They all say its an air pocket. Cause head gasket test came up negative. They bleed the system drive it for five miles and say its fixed. The next day. Back to the drawing board. If its not the head gasket or any loose hoses or the radiator cap (those have all been ruled out) any ideas what else it could be that would cause continued air pockets ONLY after the car has cooled down and started again. Thanks I really appreciate your help im just at a lost when the dealership doesn't even know whats wrong probably cause they don't want to spend the time to find out. Thanks again any insight would be greatly appreciated