Pop that cap off, there is a rubber O-ring that is cracked. If the whole plastic part inside is cracked, then replace it with a rebuild kit. Most times it is a rubber O-ring that has cracked and split apart; a common situation with rubber products in plumbing. Produce a manufacture's name and I can probably find you a PDF explosion of your product so that you know how to take it apart. It is like a floating check valve that prevents the reversal of flow from the hose connected to the faucet, back into the potable water system.