Toilet tank flap valve problems.

I have a strange problem that was not solved with the simplest approach.
The toilet tank in bathroom would leak water into toilet (not to floor) and refill periodically, which was noisy and obviously wasted water.
So, I went to a store and bought a new "flapper valve". Unfortunately, it made matters even worse: the new one leaked water so badly that the tank would not even fill! (we also have water pressure issues)
As a stopgap measure, I added a 7/8" nut as a weight and that seems to work to stop leaking -- but the toilet does not flush as intended as one needs to hold the handle to continue flushing (ie, the valve does not lock in the "up" position util the tank drains, as it should).
Without the nut, if I hold the valve with my finger until the tank fills, the valve would continue leaking a little bit after I remove the finger. The extra weight of the nut stops that leak.
So, I suspect that perhaps the rubbery ring that the flapper valve seals, is bad. Does my susppicion make sense? An reasonable course of action?
i
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see: www.fluidmaster.com
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It sounds like either the seat (what the flapper closes on) of the flush valve is worn or you may have a small leak in the flush valve gasket. They have a kit at the Home centers to glue a new seat on top of the old one (make sure it is very dry before you install or just replace the entire flush valve assembly (including the gasket on the flush valve), kit for this is also available. Don't over tighten the gasket as it will then warp in short time and you will be right back to where you started.
Either bring with you pictures and measurements or bring in old part to insure you get the right kit the first time.
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Thanks to you and buffalo bill, I think that I will try a fluidmaster 555C kit, that has a valve seat and everything. .
i

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Ignoramus5876 wrote:

Had a very similar problem with ours. Turned out the flapper valve didn't fit properly, and we couldn't find a replacement unit that did. So I trimmed the new valve with a razor knife to make it fit.
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