just changed my thermostat [1999 seville]per my mechanic. car was overheating so i had it checked for the head gaskets none blown thus far. was told to change thermo so did, then was told to bleed out the air by running car with cap off and heat on. tried to do so but still seems like there is an air pocket in there. after car gets hot hot air starts to blow out cold then antifreeze boils over. thought i fixed yesturday ran fine for 1.5 hours. today did the same thing. bought a new cap from de