I have a gas boiler that continuously leaks from an overflow pipe. The expansion tank is a long green steel tank hanging from the floor joints. I believe the issue with the leak is that the expansion tank is full with water and no longer has the proper air to water ratio. We tried draining the water from the expansion tank but a constant flow of water from the boiler automatically refills the tank. I believe we need to drain the tank, and re-equalize the air level as well. How do I do this without the expansion tank automatically refilling? There is a valve at the inflow pipe of the tank and a special opener that I found. Is this involved in the tank drain?