Long story short:

Bought the car with a leaky rear caliper, replaced all four calipers/discs/pads.

Drove the car for a while, then the master cylinder started leaking - brake fluid was slowly dripping down the booster.

Replaced the master cylinder, bench bled first "normally," then secondly by gently forcing brake fluid through each of the output ports. Installed it, no problems.

Decided to connect up the pressure differential switch - it was disconnected when I got the car. Now, the BRAKE light is on all the time. If I unplug the differential switch, the light goes out, and works normally with the handbrake. I tried two switches (both used) that behave properly out of the car - they are open at rest, and short when the pin is pushed in.

Braking seems normal/good.

I read a post here that said the switch should only light when the pedal is applied, since it works on differential pressures.

I read another post here that said if it goes on once, it will stay on until the fault is fixed and normal brake pressure is applied resetting the light.

Can anyone say for certain how the light/switch behaves? I'd like to sure or whether I've got a bad prop valve, bad switch, or an actual hydraulic fault...

Thanks!