Thanks, Alex. I didn't know about the black tag MC system.

"Incidentally, the diagonal split warning light is a bit late as you're usually mid-spin when it comes on ! "
 So, by that it is really a spin indicator light. Glad I have a yellow tag system. I don't need a warning light to tell me when I'm in a spin. (My wife usually informs me!)

Close.

Depending on the age of the car and the type of master cylinder fitted, the warning light does one of two things; it either warns you of low brake fluid (yellow tag MC) or tells you that the split braking circuit has been enabled (black tag).  The switch is there merely to be able to check the operation of the light.
Incidentally, the diagonal split warning light is a bit late as you're usually mid-spin when it comes on !

The brake check button has a warning light in it. When your brake fluid gets too low, the warning light comes on. The switch is merely to test the light bulb. Weird but true.

How and what does the brake Check button on the dash do????