anti-syphon loop repair

[gettinthere]

I have an anti-syphon loop for my shower sump pump. It is brass of unknown origin, circa 1985. It has a screw-on cap with hole in it. The U shaped fitting looks to be 3/4". Local chandlery does not have parts. The old rubber thingy inside fell apart and needs replacement. I have several more of these on the boat. I'd like to find the rubber replacement part instead of replacing the whole loop. Local plumbing supply house said good luck. Defender doesn't have anything.
Where do I find this little rubber part for the anti-syphon loop???

[Roy M]

Cut a piece of innertube rubber the size of the opening, then punch a hole through it with a sail needle. Insert, screw down the cap, you're done!