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Can you post a picture of the vacuum breaker? click on ADD FILES next to the SEND button to post a picture here
The vacuum breaker can be repaired or replaced but I need to see what you have to make a specific recommendation
This is the closest I could find to what your picture looks like. I'd suggest you go to a local plumbing supply house that sells Kohler with this part number and your pics and see if they can order parts for your unit
Given the vacuum breaker you have, repairing it (if you can get parts) is the best option. If you can't get parts then a plumber can replace it. There is an internal gasket inside the vac that should seal when water is being run or immediately after. Given the age of this part it's not surprising that this gasket has worn out and is starting to leak. The purpose of the vac is to prevent water from siphoning out of a full tub back into your water piping where it could contaminate the water you drink. The circumstances have to be just right for this back flow to happen but the code requires a vac to prevent it from happening at all.