Sounds like the O-ring on the water valve is leaking. You will have to remove the pedal, probably best to replace it while you are at it, then you will have access to the water valve. How old is this toilet? Still under warranty? If you have crusty buildup behind the pedal, this has been leaking for a while. You may find it easier to just pull the toilet to make repairs, two nuts holding it to the floor flange, and the water inlet fitting. The kits for the pedal and the water valve both include a floor gasket. I had so much trouble with ours, that I just replaced it with a Dometic 310. So far, it seems to be a much better design. Good luck. Parts list at
http://www.thetford.com/HOME/PARTS/P...2/Default.aspx