I put a volt meter on the brake switch with the brake pedal depressed and got no reading.
To read voltage at the switch, one voltmeter lead needs to be on a proven chassis ground, and the other to one of the two switch terminals.
One switch terminal is hot, probably only with key in ON position. The second terminal position will become hot if the fluid pressure closes the internal contacts.
The switch is not a common VW unit, as some believe. But is sold by our vendors.
I just replaced mine, as I needed mucho pedal pressure before the lights went on. I did it without any problems, and did not bleed brakes afterwards.
Cost less than $20 from Wilkinson.