[timsullivan]

Yes, you're right, the plastic overflow tank mounted on the right side fender well undoubtedly is the only source for the cooling system to literally and figuratively blow off some steam. Most likely you have the expansion tank (that sits directly above and in front of your thermostat housing) overfilled. They should only be filled to the midpoint of the expansion tank when the car is cold. That or you have some very serious overheating problems going on. Normal cooling system pressure relief shouldn't see water coming out of that very tiny breather hole on top of the overflow tank unless you have the puppy filled to the brim! Drain it all out of the oveflow tank and reset your coolant level in the expansion tank to the midpoint.

The valve on the bottom of the overflow tank should only move about 180 degrees to open or close the drain. If memory serves me it's just a press fit into the bottom of the tank.