Buy the new rubber valve that fits in the cap. The old ones disintegrate over time and turn to black goo. The part number is 53766A. They work in most of the Mac caps I have ever worked on but, not all. If your 10-10 caps are original, they should work if the rest of the parts are there. If you don't want to gamble, I would suggest you look for caps on eBay. Try Bob's Lawnmower Service @ (607) 638-9297, the valves are $6.00ea and are made of a more current compound that won't disintegrate as quickly with modern fuel. He might have complete caps also, not sure, he was running low about a year ago when I last purchased one.