It is possible to repair, but quite difficult. There are small, delicate plastic parts inside that crack and fail, causing this problem. Getting the switch apart is not easy - the body is cast zinc which also cracks easily. I went to a lot of work to repair one, only to have it fail again. Attached some pics so you can see what is inside. Notice the crack in the little part near the pencil; that is the root cause.
Finally I bought a later model Lucas part off ebay. It required a slight mod to the mount bracket, splicing wires into the stock Pantera connector, and making a new cancel cam.