Just like the title says, and I can't seem to get them out completely. Over the winter, I accidentally left water in the system and it popped out some weak freeze plugs that had actually been leaking for a while. There was a trail underneath both of them where water had been coming out and it stained the block. Ive replaced those freeze plugs, the radiator and hoses, water pump, thermostat, and heater core. I basically have a new cooling system from the ground up. The head gaskets have 6000 miles on them and the intake gaskets have 1000 miles. There's no oil and water mix, or oil in the radiator. The coolant pressure tests good, at 16 psi, it doesn't move for more than a half hour. I've tried bleeding the way I learned and have always done it, I've tried jacking the front end up and bleeding. This method works only for a few days before the air comes back. I don't know what else to look for. Can someone please help me? Thanks