I sometimes recover them by applying a very high voltage across them, whilst operating them hydraulically. Once the spark across the contact gap cleans away some of the contact corrosion, they often come back to life again. Then once a resistance is built up, by repeatedly switching a couple of amps through them from an inductive load (e.g. like the solenoid start switch coil) they gradually build up a very low resistance again.
However if they have leaked hydraulic oil inside they are generally fooked. Same goes for engine oil pressure switch and similarly for 400/4 etc horns.
Also plenty around used, as stated previously.