Hi Robin,

I agree that sounds like a reed-switch...

#3.. The BOSCH Rexroth website has a handy multi-language media search and a quick browse suggests that a 'hydraulic interruptor' is in fact a float switch - I would guess to check the level of hydraulic fluid - or it could possibly be a pressure switch. They all seem to be conventional contact switches so I doubt you'll do any damage by probing with a multimeter.. I would guess its a series connected 'AND' gate of switches...