I second changing the bearings. They're only $8.97 a set:
http://www.rc-bearings.com/catalog/p...a3ebe33959523b
Wonder what a new sleeve and piston would cost
if you could find one after those old bearings fail?
Dawn Power Dissolver and an old tooth brush will remove any old castor gunk:
http://www.dawn-dish.com/en_US/powerdissolver.do
David