American Standard Toilet Seats How Hard Is To To Replace Just The Little Rubber Cap In A Standard American Toilet Tank?

How hard is to to replace just the little rubber cap in a standard American toilet tank? - american standard toilet seats

I mean whatsit Flippy black rubber in the water tank from the top and bottom removed, flip the lever bathroom. It works more or less, but seem to have hardened, so that if one falls, the seats are not the most appropriate and should be pulled down manually with a small force, maneuver, however, keep a little of the water seeps into an astronomical water bill if nothing is done, not done without each time.

Where is obtained as a spare anyway? How? What is called the part?