breagles

I had a problem today . When I went to the store I came back out and coolant was pouring out of the overflow tank cap . The first thing I thought was a thermostat , so I bought a thermostat and a coolant resivor cap . I am getting ready to replace now , but my question is could this be a result of an air pocket in the engine ? The cooling fan is coming on like normal and the car is overheating at idle . Another question is there a way to bleed this system ? I just replaced the motor and I did not see any other way except running the car with the resivor cap off . Any help will be appreciated .