Not long after we moved into our 50s-era split-level about 30 years ago, both toilets failed. The culprit was the cheap plastic ballcock valves and I installed all-brass valves at maybe $15 each. Everything worked well until 4 or 5 years ago, when they started not quite shutting off. I bent the float rods down and that did the trick for a few months. Then the leakage started again, I bent the rods some more, etc. Finally replaced them both 2 days ago with cheap plastic Fluidmaster 400s. No float rods to bend, at least. The brass valve's float lever presses against a ceramic button which pushes an internal rubber "cookie" against a perforated disc at the inlet. It's quite ingenious, but the cookie had started disintegrating with age, so the ballcock lever had no chance of stopping all leakage no matter how hard it pressed down. I might add that all the brass components were in perfect shape. Maybe with a more robust formulation of synthetic rubber for the cookie...who knows?