> >In the braking
> >system is a hydraulic/electrical switch device with a piston in it. It is
> >fed brake fluid from both circuits to opposite ends of the piston. So while
> >everything is working properly than you apply the brakes the pressure on
> >both ends of the piston will be equal, and it will do nothing. If either
> >brake circuit fails the pressure becomes unequal and the piston moves,
> >completing the electric circuit and lighting the warning lamp.
>
> At least in my '72 Karmann Ghia (and I believe in most 2-circuit VW's),
> this is not the case.
>
> There are two switches, one on each circuit.  These switches are used to
> light the brake lights in the back, and also work to light the brake
> warning light when there's something wrong.  Each switch has 3 prongs,
> the inside of the switch looks like:
>
> 1   ___        ___   2
>        \      /
>             /
>            /-----<pressure<--------
>           |
>           |
>           3
>
> (note: these numbers are not meant to correspond with the numbers on the
> switch!)
>
> When there is insufficient pressure on the switch, it connects 3&2
> together, when there is enough pressure it connects 3&1 together.  You
> want +12 on 1, the warning light on 2, and the brake lights on 3.
> This way, your brake lights come on when you have enough pressure (1&3
> are connected), and your warning light only comes on when one has
> enough pressure (1&3 are connected) and the other doesn't (3&2 are
> connected) so it completes the circuit to the warning light.
>
>                        warning light
>                           |
>                           |
>                   ________|______________________________________
>                   |                                             |
> +12 ___        ___|                         +12 ___          ___|
>        \      /                                    \        /
>          \                                                /
>           \  enough pressure                             / insuff. pressure
>            |                                            |
>            |                                            |
>            |____________________________________________|
>            |
>            | brake lt.
>
> You can see how this circuit to the warning light connects fully to +12 only
> when one cylinder has sufficient pressure and another doesn't.
>
> In 1972, VW added an automatic lamp test to the brake lamp, by splicing
> it off of the generator light.  People often seem to want to rewire stuff
> so that the lamp test works but then stays on all the time, which might
> explain why your light was on all the time.