1. There should be a little free-play in the master cylinder rod.
2. The brake shoes should not be touching the drum after a proper adjustment. Usually, you adjust the shoes until they just touch the drum and then back off 5 clicks.
These procedures are described in the GMC Maintenance Manual (available from reproduction parts vendors on onl-line and on eBay).
#2 is at least part, if not all, of your problem.
Tim
