Sonny's Tire Shiner Continuous Drip

We have a tire shiner that is several years old. Recently it seems to be constantly dripping product out through the pads. Has anyone else experienced this? Where should I look for the problem?

Sounds like the check valves are bad.

I feel dumb. I figured check valves but when I looked at the machine I didn't see any. After your post, I looked again and found the check valves this time. They are bad, that was the problem. Thanks.