Check out the link, it should help.
Also, you might want to try a test to see which tube(*) is causing the problem. Take some water and pour it into the gutter tray. You should see water draining out in back of the front wheels and in front of the back wheels. Whichever side you don't see water pouring from, thats the side with trouble. Unless the tube is only partially disconnected or plugged, then you'll have to see which side is draining less than the others.