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?