96 century coolant problem

The heater on my 96 Buick 3.1 started blowing cool air so I checked the antifreeze level and found the overflow tank was full. The rad was half empty. Trying to correct this problem I have replaced the thermastat, flushed the cooling system, had Rad cleaned, new cap, have new water pump. I pressure tested the cooling system and no drop after 15 min. The car runs fine but something is pressurizing tha coolant system. I have bled the all the air out of the system. There is no coolant in the oil or oil in the coolant. There is no noticable loss of coolant (it all seems to go into the over flow. Any suggestions Thanks

The heater on my 96 Buick 3.1 started blowing cool air so I checked the antifreeze level and found the overflow tank was full. The rad was half empty. Trying to correct this problem I have replaced the thermastat, flushed the cooling system, had Rad cleaned, new cap, have new water pump. I pressure tested the cooling system and no drop after 15 min. The car runs fine but something is pressurizing tha coolant system. I have bled the all the air out of the system. There is no coolant in the oil or oil in the coolant. There is no noticable loss of coolant (it all seems to go into the over flow. Any suggestions Thanks
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My guess is you have a leak in the tubing between the overflow tank and the radiator cap . . A hot engine pushes antifreeze into the storage tank but when engine cools, instead of sucking antifreeze back into the engine, a leak is letting air into the radiator . .

Well I finally figured it out. The head gasket at the #2 cylinder was leaking. The way the gasket failed it only let exhaust into the water jacket. The pressure testing of the coolant system wasn't at a high enough pressure to find the leak. I'm in the process of putting it back together now. Thanks

Congratulations