Here is my experience with the brake light coming on in my 1981 Westy.

First of all the clutch master cylinder started leaking a little, and
then a LOT! So the fluid level in the reservoir may have gotten a
little low.  In any case while I was stepping on the brake pedal hard
once during this period of leaky clutch MC, the light came on.  There
is an integrated circuit that the two brake light  switches plug into,
and yes the idea is to sense the pressure differential between the the
two.  If one comes on and not the other, then this is possibly due to
a failure in one of the brake hemi-circuits, hence the light comes on.
However the reservoir was full, and a VOM test revealed that the brake
light switch would never close, even with great pressure on the brake
pedal.  Then soon after (days) the switch went wacky the other way.
It was on all the time!  This could be treacherous since the battery
would run down under continous brake lights.  I simply unplugged that
switch and it worked fine for a while.  The next weekend I replace
both switches and it has worked fine ever since.  So I think EITHER
switch is sufficient to light the brake lights, but if one is broken
or there is low pressure in one circuit the warning light will come
on.  

The dealer price was $25 each, but Kohl-Weiss had them for $6.  Guess
which ones I used?  I can only guess why the switches failed, but
maybe air entered them when the fluid level was low and somehow
caused the problems, or maybe just "excessive pedal pressure"?  Who
knows.  The replacement was easy.  I filled the little indent in each
switch with brake fluid, filled up the reservoir, and quickly replaced
the switch before too much fluid leaked out.  

By the way, rebuilding the clutch master cylinder was incredibly easy,
after about $14 in new rubber parts.

Of course, the new front rotors were a different story at $75 each!