: : Ironic that just today I am looking for an answer to the exact same question. Its been a problem for some time and we've just been using other faucets. With some landscapping we have moved the hose box thing over to near that faucet so need to get it working again. The top of the faucet says Mansfield Anti-siphon. It has a grey cover and when unscrewed shows a white piece inside with a tiny rubber washer. You can block the water from the top with the cap off and prevent leakage so the key seems to get that washer sealed properly. Despite all attempts I can't seem to get it sealed.

: : Any help would be appreciated. Thanks in advance.

: I've had the same problem on 3 of 4 sillcocks at my home. I have 500 series sillcocks from Mansfield. I wish I could replace all of them. Instead, I replace the washers periodically with a Mansfield Sillcock Service kit number 630-7755. This kit contains replacement parts for 300 and 500 series Mansfield sillcocks and has fixed the severe leakage problems I was having. I'm trying to find the kit online since it costs $10.45 at a local plumbing supply. Please reply with a source and price if you find one on the Net. Thanks.