nicktheplumber
The water in the tank should not reach the level of the overflow pipe (what you call the “mast”). That’s your problem. Whatever kind of float-valve you have in your toilet should shut off the water flow into the tank when the tank water level is within about 1″-1/2″ of the top of the overflow pipe. Either the float needs to be adjusted or the float actuated cut-off valve is leaky. Try simple adjustments, and if that doesn’t work, replace the float valve as a unit. I like the Flo-Master units…they’re cheap, easy to install, and work great.
