It's not going backwards, it's not closing during the rinse cycle. Common item. Caused by a build up is calcium. Remove the top and core. soak them in vinegar or other food grade acid to dissolve the minerals that are making it hang.
Thanks fixbear. Cleaned all up with vinegar, but still not working. Ectofix, yes, water coming from the vacuum breaker. Bought a replacement kit, and found they are supposed to have a spring. It does not come with a spring retainer and our old one doesn't even have a spring. Going to have to buy the whole vacuum breaker. Olivero, yes, I think there is a rinse aide. Never paid attention as I don't use it myself. Just a maintenance guy trying to save the business from a plumber fee.
Okay, well, one thing I ran into on my Stero machine is the rinse aide was actually very corrosive, so the standard vacuum breaker on the rinse aide line would get eaten rather rapidly, I had to change it about once a year, the brass would just get eaten. It would also eat the copper pipe it was run in for about 1′ it would just eat through the elbow.
Olivero, that sounds like electrolysis instead of corrosion. But I'm also surprised that the injection is upstream of the vacuum breaker. The purpose of it is for the protection of the domestic water system contamination. As is the incoming water check valve. I've had zoning inspectors check that in the past. Being the tap is often made by the chemical supply company installers.
@grasshopper, When you took it apart, did you check the seats to be smooth and verify that the guide post do not hang? This is a gravity valve that has to be plumb to work right. The moving valve part also uses air to help it float with the water rush in. You may also want to check the water inlet line screen before the solenoid valve for restriction. And check the water line pressure. If the water coming in does not come fast enough, and at the proper pressure, the valve will not seat. I've seen were a supply line got partially crushed from slamming racks under the drying table. You also want to check the rinse spray wands to have tight bearings and all of the nozzle's in place. We all tend to forget that other things may effect what we see as a problem.