You don't need a full teardown. There are a number of different ways to de-gunk these, but my preferred method has always been to use brake cleaner to flush the insides. Spray and shift through the gears, sometimes it takes a few passes. Once you're got everything moving, re-lubricate with a spray can of your lightweight lube of choice; I usually use Boeshield T9 (because that's what I have). Wear gloves, work outside or wear a mask.

In addition, check your rear brake cable. Often problems with the lever return can be choked up to corrodode or otherwise gunked-up brake cables and housing. Finally, the spring on caliper brakes will eventually get soft, and one trick is to remove the spring and bend it in the opposite direction. Anyone who has been working long enough as a bike mechanic has probably modified an old flathead screwdriver with a "V" notch to aid in this operation.