Is the small piston that begins to leak oil located at the top side of the windshield that releases the convertible top on an Mercedes SL500 able to be rebuilt Is so by whom?

I wouldn't think so. New part is to cheap, it is cheaper and safer to buy new. Total replacement, including installation should not cost you both sides $400

New is NOT cheap and rebuilt can be better than new. Depending on the year of your SL500 you may have either 11 or 12 hydraulic units which power your soft top. Top Hydraulics, Inc. in Oregon rebuilt all 11 of mine for $550. Klaus, the owner, claims his replacement seals will last longer than the original MB seals. His after the sale customer service is outstanding. Hope this helps.