An over flow tank is a catch tank for the over flow valve on the radiator. There is no pressure involved on such a tank. It is of marginal use on most vehicles. Many vehicles have reservoirs today that are integral to the cooling system and the cooling system is filled through the reservoir. Such a reservoir has a fill line and allows for expansion, eliminating the need for an over flow tank.
If you can’t get the guard off it will be difficult to see the leak. You might consider getting into a shop soon. And with the high probability of engine damage it would not be advisable to drive far.