A coupe years ago I contaminated about three batches before I discovered my ball valve was the culprit. Mine was a two piece stainless and boiling the whole thing for 15 minutes before putting it on the kettle did a great job. I did this for about a year.
These days I use a the Blichman three piece stainless ball valve and I take it off the kettle, disassemble it and soak in PBW between every brew session. I use the hottest tap water and PBW does the rest. I let it soak for three or four days before rinsing and there is NOTHING left. PBW is fantastic.
Anyway, if you plan to stick with brass I would soak it in super hot water and PBW with it open, remove and put the valve at half open and soak again, then close all the way and soak again. Rinse thoroughly with hot water later and move that valve around a lot to clean out all the residual PBW. That should do the trick and boiling the wort should add enough heat to kill any funky bugs.
Good luck and cheers!