The vacuum switch on the brake& clutch pedal(s) disengages the 4wd when braking (or changing gear) so if you lock a (rear) wheel the system doesn’t try to engage 4wd and push power to the locked wheel thus overloading and straining the drive train,
the one on the clutch pedal also disengages 4wd when the clutch is depressed, if you have ABS then you dont have the vacuum switch on the brake pedal (its not needed as theoretically you cant lock the wheels) drive in reverse is sorted out by the gearbox (electrical) switch
I have a syncro running a hydro clutch set-up put on by the last owner. I have had wiring issues and under the dash, tucked away is a white plug (two wires) that is just stashed doing nothing (car is not ABS). Question is after reading this, should I be worried by this or not and would I be right in thinking that I should look at getting this refitted? Does this switch activate by being pressed or allowed to extend? Obviously extended right now.