I have an 820 and front window on cabover doesnt leak on mine in pouring rain up to 75 mph. One thing about front window, it does collect leaves so if youre going to clean leaves dont let one get between window and gasket. It will leak. Ive since put another half round gasket around original gasket. The little knee window does leak. I have a long bed truck and short bed camper so im thinking air coming between truck and camper is hitting it at whatever highway speed im at. I used hose at house and sprayed all over it looking for leak but nothing. I have some rubberized canvas and sticky backed velcro to cover window. We'll see if that helps. It only seems to leak up from bottom of the fixed piece of glass. Like its being forced thru sliding window gasket. Its not a huge leak by any means.