The red light on the dash with a B imprinted on it is the dual circuit brake warning light. It is a simple "and" logic that measures voltage from both of the brake light switches on the master cylinder. If one is supplying voltage (is working) and the other is not (is not working) then the light comes on to let you know that half your brakes have failed. If the light comes on when you apply the brakes, it is definitely time to stop driving and fix something! Especially if it is a brief flash as the brakes are engaged!

On different years of bus there are three different bulb tests that are applied. On some years, you press the B and it lights, on some years the bulb comes on when you start the car and goes off when the car starts (along with the gen light), and on some years the light comes on when the parking brake is engaged.

Personally, I don't think much of this warning light, as the inability to tell when half the brakes are gone is not a widespread problem. Believe me, you will know! Now maybe King Kong would not be able to tell, but you and I will definitely notice :-) The time to notice, tho is when it is flashing as you apply the brakes. this *may* mean that there is a leak within the master cylinder between the circuits and that one circuit is failing but the leak is feeding fluid that keeps you from noticing. If it flashes, stop and fix it.