water coming out of top of expansion tank

[Nadine]

So i have done a search and i think i found the answer but wanted to ask as my symptoms are a little bit different. So i have no water in the oil, and no oil in the water, car does not over heat and fans kick in when they should. However when ever i top the car up with water and then leave her to run when i switch her off once she has reached temp, water starts to piss out of the top of the expansion tank. It doesnt come out the overflow pipe, just the top of the tank. Now in my search people were suggestion its the rad cap? is this right?