Sounds like you have a couple of problems. The radiator cap should release at 13psi. If it doesn't you can build up enough pressure to blow a hose or radiator. Sounds like you already breached the radiator. Firstly, get a new radiator cap. Then investigate why this is happening. Starting with a cool engine, remove the radiator cap and start the car and fill with a 50/50 solution of water (distilled is best) and coolent. Keep topping off the coolent until you don't need to add any more. If you have a blown head gasket, coolent will gush out of the raditor and you won't be able to fill it properly. When the coolent is topped up (and assuming it's not gushing out), replace the radiator cap and test drive it. It's normal for the coolent to come out of the radiator through the cap and go into the overflow reservoir. After you shut the engine off and it cools, coolent is drawn back into the system. Judging from your other comments, I suspect you'll need to replace the radiator too.