Pressure Differential Warning Light.

Can the above electrical switch on top of the units be changed without the
loss of fluid.
Is it a case of just changing a switch not disturbing the brake fluid line.

 

You can just reset the switch as I recall, just remove it and put it back in.
Very often, after a pressure imbalance occurs, the whole junction box needs to
be replaced since the O rings on the pistons get mauled by travelling into
normally unused cylinder territory (which has become abrasive and corroded
from disuse) when an imbalance occurs. The symptom is frequent relights of the
warning light after resetting the switch.