Do not use Coke to clean a WC'ing setup since it contains sugars that grow the green funny things in the water.
50% Distilled White Vinegar and 50% Distiller Water mixture is the start. If the tubes are really bad, then replace them.
Should I flush a new system with Vinegar?
http://forums.procooling.com/vbb/showthread.php?t=12167
DO NOT use "regular vinegar" since that is made from apples. Apples contain high levels of sugars. You'll be feeding the funny green things and they will multiply.