Anti-siphon hose bib

All my antisiphon hose bibs are leaking badly when water is turned on; water pours from the top where the green plastic cover is. I understand that there is possibly a leaky washer underneath, but I can't figure out how to get the plastic top off. (Hexagonal green plastic). Brute force, or is there a trick?

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.

the ones ,i,ve seen in Ca.

are metal. sound,s better than plastic

I have found that many times it is just debris that can be clean out.