Rubber flange on the back side of a Prier outdoor mixing stem

[Rslaback]

So some idiot who looks a lot like me left the hose on the C-108 faucet on the outside of our house this winter and as expected it split the side of the coper extension that goes back into the house. I figure it is worth giving a solder repair a try but I noticed that it tore the back of a rubber piece on the back of the stem.

Anyone have an idea what the purpose of that rubber flange is? I've never see a piece like this on any other outside hydrants.

 

[GReynolds929]

It's part of the anti siphon

 

[Sponsor]

 

[Rslaback]

So basically an easily defeatable rubber check valve to prevent anything in the faucet from making it to the water supply?