The toilet tank was leaking water, so I replaced the rubber gasket at the bottom of the drain valve with a new identical one (size and shape). However, the toilet is still running. If I manually press down the drain valve really hard, then the leaking stops.
Nothing seems to be wrong with the valve itself, it can move up and down without any problems. (* Well, it WAS ok until I accidentally broke it while experimenting :S)
I noticed, however, that the new gasket already accumulated some limescale. Is it possible that the fix counterpart of the gasket should be replaced as well? Is that even possible without replacing the whole tank? (It seems to me not possible to remove it at all.)
In other words, I should decide if I need a new valve or a whole new tank? (Still in other words: can it be that replacing the gasket is not enough, but replacing the valve is?)