Switch off the supply of water to the cistern. With a WC, it will be a little easier to work if the cistern is flushed to remove the water.
There is a split pin holding the float arm in place. This should be straightened and removed to release the float arm.
Removing the ball valve washer
Unscrew the end cap if there is one, to expose the piston. Because the whole mechanism is subjected to constant moisture, you may find the end cap difficult to unscrew. Try gripping the body of the valve with a pair of pipe grips while unscrewing the cap with another pair. If needs be, add a little penetrating oil at the cap/thread junctionand leave to act for a few minutes so that the oil may creep along the thread.<
Slide out the piston and remove the old washer from the end.
Replacing the ball valve washer
When fitting the new one, gently remove limescale deposits from the assembly with wire wool or a pan scourer.
Then apply a little petroleum jelly to keep the parts from seizing in future.
Reassemble the piston in its housing and reconnect the float arm with the split pin. Once through, the tails of the pin should be eased apart to hold it in place.
Turn on the water and check that the valve functions correctly – cutting off the water supply before it reaches the overflow.