Most of these old style (and I think many of the current ones) use a type of spongy material to spring the contacts back open when you release the grip on the switch. This old foam (I call it foam for lack or a better word) deteriorates with time and will no long springs back to its undepressed state, in fact it turns to a powdery form and just falls apart. I have temporarioly restored them to use by finding a suitable springy material to place between the contacts in liu of the old stuff. You could also install a bypass switch if you can find one with adequate capacity (at least a 20 amp capacity). You do need a very good switch for it to last, also very heavy wire on the switch contacts.