The plastic sleeve just makes it easier for the arms to slide over the spring, if it wasn't there, there would be more friction on that joint. If it's worn-out, you can typically twist the plastic on the spring and slide it down slightly so that the arms rub on a fresh portion. Add some teflon/graphite/moly grease to that point.
If the calipers have been taken apart, the pivot-bolts may have been tightened down improperly. Or the incorrect stack of washers was installed, leading to binding. Also make sure to add some long-lasting grease to the pivot-bolts and on the caliper-arm's mating surfaces when re-assembling.
Also, does the calipers expand by themselves with the cable disconnected? If you disconnect the cable and squeeze the calipers and find that they expand by themselves, excessive cable-friction is the problem. I've found that the rolled Diacompe cables to work the smoothest and last the longest (add a little teflon grease to the inner cable when assembling).