I have a second hand prestige cornet. The second valve returns slowly especially if held down for a few beats. I have had it to 3 differernt instrument repairers. The valves all appear to fit quite snugly and will pass into the bottom of the cases. The valve is slow also in the other cases and the other valves work ok in the second case, so I suspect there is something wacko with the second valve! It often has a dark sludge on it when I clean it. I have cleaned the inside of the instrument ad nauseum, with soaks in the bath, the snake, everything I can think of! I even polished the valves and cases with metal polish, cleaned with a bottle brush and detergent, even tried acetone and 600 grit emery paper, then polish again! Tried several valve oils, Al Cass, Dennis Wick, and Pure. I'm becomming emotionally destraught praying fervently with quivering lips and beating heart! Can someone please tell me what the dickens is going on here, or how to fix it?