I would remove the gas tank and clean that out too, since that's what is making your petcock dirty--rust chunks in the gas tank. Search on here for gas tank cleaning or de-rusting. The easiest way is to take the tank off, fill it with coke cola, shake it up and let it sit overnight. When you drain the coke out you must fill it up with gas/oil mix immediately as it will rust within a half hour if left empty. Good luck.