it also lights up should you have a brake fluid loss as well as the handbrake on warning light (which is why it connects to the dual master cylinder)
^this. If fitted, lights up if a brake circuit fails and may also act as a handbrake warning light. A blanking plug was fitted if the van wasn't equipped with the warning circuit.
That is a good idea I might do that with mine as pretty sure I don't have brake warming light switch in the master cylinder!