I have just had a new frost free sillcock installed. It leaks intermittantly from where the plastic faucet is joined to the metal pipe that sticks out from the house. This happens mostly when I use a garden hose with a sprayer nozzle attached to it and I shut the water off via the sprayer nozzle. I have had some people tell me that this is suppose to happen and has something to do with an "anti-backflow" feature. True?