Keith,

The Skymaster wheels have an O ring in a shallow groove that seals the 2 wheel halves. If the wheel has ever been pulled apart to release air, there is a good possibility that during re-assembly the O ring was not seated in the groove and was pinched. It doesn't take much to damage the O rings to the point where there will be an air leak. I do not know the wheel size that you're working with but the O ring size for the F-4 is 1mm x 50mm.