Your C20 has full floating rear axle so your guy is likely correct, you'll need to pull the axle. It's an easy DIY job. Personally, I'd buy a new wheel cylinder, they're not that expensive. But if money is really tight, a rebuild is OK too.
Check this thread, it might help:
http://67-72chevytrucks.com/vboard/s...d.php?t=660022