It was, IIRC, years into his imprisonment before his captors found out he was the son of the highest ranking admiral in the US pacific fleet at that time. Months of torture did not prompt him to give up that info. Once his captors discovered his relation to the US admiral, they treated him better but did not cease torturing him (essentially, his living conditions were a form of constant torture in any event, as is the case with folks at Gitmo, despite this PR crap we hear about how good they have it, they gain weight on luscious meals, etc.). So according to McCain's own writings, torture failed.
All the more reason he's a craven bastard for being against the anti-torture, anti-waterboarding bill.
-B.