I am using my K-500 to clean some roof drains in Chicago. We popped a drain and we heard a gulping like noise, and we could see water splashing at the 90, about 10" down. We didn't have enough hose to run it directly into the drain, but kept filling and dumping 5 gallon buckets, all the water would drain, and I got about 35 foot of 1-1/4" cable down, so I was actually under the slab. I go inside and try to spin the brass cleanout plug off, no go. So I get the hammer and chisel, I break the cover and a hiss of air rushes out. I about crapped! It kept on hissing, and never stopped. I looked and discovered a 3/4" galvanized line coming over from the paint booth is tapped into the roof drain, right below the double wye. WTF!? The plant maintenance boss has no idea what it is, we don't know if it's condensate or a pressure relief drop or what.