Have you checked the thermostat?
Is the coolant in the radiator staying at the same level, or are you constantly topping it up?
The only reason for the expansion tank to be full and overflowing is if the head gasket has blown and is pressuring the coolant system, which results in loss of coolant through the expansion tank.
P.S. yes the expansion tank is connected to that part of the frame by a small rubber tube. it lives up behind the side panel above the rear wheel. there's a min & max level about halfway up the tank. Check it out to see if it's full to the top...