Griff in Fairbanks wrote:
Are you talking the brake lights in the rear or the brake light in the dash?
If you're talking the brake light in dash, it's the 'brake failure' switch located on the proportioning valve. It senses the lose of pressure in the front or rear brake systems via a piston 'balanced' between the front and rear systems.
Sometimes this piston gets 'off-center,' yielding false readings. Alternatively, the switch is a normally closed switch, held open by a bump in the piston. If the switch is broken or somehow stuck in the closed position, you'll also experience 'false positives.'
This is the same type of switch, and same function, as the neutral start switch on your automatic transmission. The same with the switches on car and truck doors for interior lights,
(Modern vehicles use magnetic reed or proximity switches to sense 'door ajar.')
I’m betting rear brake lights. I didn’t realize the MB300 didn’t have a return spring, until I saw the rear brake lights on. The brake switch doesn’t require hardly any pressure to activate.
