I was in my office when I heard running water. Rushed across the hall into husband's bathroom, where I saw a flood. The tank of his toilet was overflowing. Took off the lid and pulled up on floater ball thing--water didn't stop from the outlet tube even though water was flowing over the top of the tank.

Turned off water to the toilet. Water quickly flowed down the toilet into the bowl, and all water levels steadied. Could this be a value problem or something else? The toilet had not been flushed in at least 6 hours, and water had not been running continuously since that flush.

Thanks in advance for any help.

Definitely sounds like a fill valve that has failed. Try it again but stay right there to watch and see if the float isn't able to stop the flow, if not replace the fill valve with a Fluidmaster 400A. I am very surprised the water level went higher than the overflow tube. That tube should be all that's needed to prevent a flood.

Bob makes a good point. Check the overflow tube, (see image) to be sure it's clear. That's its only function. To prevent the tank from overflowing. And replace that ballcock. Good luck, Tom

Your overflow tube MUST be shorter than holes in the back of toilet tank. If it is higher then - in case of fill valve malfunction - water fill overflow out of the tank and on bathroom floor.

I have decided to replace the fill valve with a Flushmaster, but I can't disconnect the water supply. When I turn the nut at the bottom of the toilet (connected to the solid water supply line), the entire fill valve turns too. The fill valve has gotten looser and some water leaked from the tank to the floor (the water I couldn't get up with a sponge), but the plastic nut doesn't seem to be going anywhere. Any advice?

You need to get down on your knees and low. Then, you need to use 2 sets of channel locks. Grab the Fill Valve stem with one pair and grab the nut from water supply with the other set . Turn in opposite direction.

Don't forget to turn water off at the angle stop.

You may not have enough room for two pliers on underside of toilet tank. You may have to grab the bottom portion of the fill vavle(in the tank) with one set of pump plier, and then loosen the supply nut on the bottom side of tank(outside the tank) with another pair of pliers. I would suggest sponging all water out of tank before you remove the fill valve, this way there is no chance of water leaking onto floor. You said you already had the water to tank shut off, so you should be fine there.

Thanks for the advice. I got the fill value out! Of course now, I'm have problems getting the tank separated from the bowl (those nuts are tight too). I'm sure I'll get it. I hope installing the new stuff is easier than getting the old stuff out.

Wait... you don't need to remove tank from bowl.. Now that you have the fill valve out, just replace the fill valve. Not need to remove tank. Im sorry if I confused you, but if have the fill vavle out, just replace it, no further work is needed here. Good luck and please let me know how things work out.

Sounds like you had a fill valve with a brass float rod and a plastic bal and the lever above the fill valve broke and therefore it didn't matter how high you lifted the ball it would not cut the water flow off. I hope by now you have replaced the valve and everything is OK.

Noooooo..! Don't remove the tank!! All can be done with tank left intact!

I hope I am not too late with this post... :=D