I have experienced this problem. For me, the fix was to lap the washers with water on a sheet of 600 wet/dry sandpaper over a piece of flat plate glass. Just go real easy so you keep the two surfaces parallel and only sand until you can see the sanding marks on both sides of the washer. I found that burrs created when the washers were manufactured were keeping them from sealing. I hope it is this easy to fix your problem.