There is a tiny hole in my 2007 model ford fiesta's coolant expansion tank. I closed it with an epoxy and everything is fine. I drive car and there is no decrease in coolant level, the problem started when i switch ON the AC.

after few kms drive with AC on, i open the hood and that hole was open and tiny bubbles were coming out of it and aprox 250ml of coolant was lost and engine starts to overheat. i want to ask does the pressure increases with the use of AC? does replacing the coolant tank will solve the problem? or with the AC on the car is drinking coolant? there are no signs of faulty head gasket. no wet spot is formed when the car is standing.