Today I finally got rid of the air bubble in my cooling system after a few months of not driving the car and was able to drive the car again without it over heating (and the heater in the car started working too). After driving it for a while everything seemed fine until I opened the expansion tank and topped off the coolant. After that, the heater was gone again, which leads me to believe there is air in the system again. I know there is a small leak underneath the expansion tank and I was wondering if this could cause air entering into the system? I know a blown head gasket is also a cause, but I'm wondering what are some other reasons for air entering the cooling system?

Thanks in advance.

 

You answered you own question.

The cooling system is a closed system... Air can only get in if there is a leak or opening in the system.

Which you stated you know there is a leak. You can used one of those emission testers in the coolant or get a compression test done to rule out head gasket.