While the brake drums are off, be sure to check for any broken return springs, and clean out the schmutz.
If your fluid level in the master cylinder reservoir is low and you can't find a leak at the rear wheel cylinders or at front calipers, check for a leak at the rear of the master cylinder (where it mounts to the booster). You can unbolt the master cylinder from the booster leaving the lines connected and gently pull it forward a little to check for fluid inside the booster.
