Been there, done that. :)
What I usually find is pitting on the brass Sloan valve. I buff it down with sandpaper by placing the sandpaper on a flat surface then moving the valve on the paper. I use three different grades of paper depending on the corrosion.
Secondly, I clean the vacuum breaker tube with "the works" and buff with scotch brite. Replace the tube if it is very bad.
Hope this helps friend.
Everything you ever wanted to know about Sloan Valves
I know this is a basic, but make sure the pinhole in the diaphram is clear.
I bet you know this, but I find that fibre washer/friction ring needs to be there or I can't get the them to seal. Also I had one that gave me fits until I found the hairline crack in the vacuum breaker tube.
Flip the valve body upside down. You will now be looking at the part of the valve body that seats on the vacuum breaker/fiber washer. You will most likely see the chrome plating chipped off or pitting with brass showing.
I made that my signature line when I told one of our customers some bad news about needing a new pop-up assembly. She just about cried. I think after 5 years of plumbing, plumbers should be given a honorary degree in counseling. :D
Zurn makes a vacuum breaker replacement that doesn't need that fibre washer.
I don't proclaim to be an expert on flushometers but I had this problem once on a urinal. As it was explained to me "the urinal is not passing water fast enough and as the valve stops allowing water through, it backs up out the vacumm breaker". I replaced the urinal using the same valve and VB, no problems.
I got a bunch of these assemblies and I don't think you can make a whole lot selling them on ebay.
I have rebuild kits, none of the electronic eye stuff though.
Make sure the urinal is clean and running properly. Remove tail piece and acid bath urinal. I also would replace the entire tailpiece assembly.