1986 Audi 5000 Repair Question
Brake warning light
Look for a combination valve with an electrical connector. You should be able to find it by following the steel lines from the master cylinder. Unplug that connector. If the light goes out, the pressure-differential valve is indicating a loss of pressure in half of the hydraulic system, typically from a leak but also possibly from a bad master cylinder.
Also unplug the connector at he master cylinder. If the light goes out, check for a level-sensing float that has sunk.
Most cars use the same light for the parking brake. Try moving the pedal or handle to the fully released position. If that works, a rear cable could be sticking applied or the main cable could be stretched and needs to be adjusted. A trick that often works on cars that use regular light bulbs for the warning light is to watch it very closely while the engine off. If it brightens just a tiny little bit when you apply the parking brake, that switch is not the cause of it staying on.
Unplug the pad sensors one at a time. If that makes the light go out, look for a bare spot on the wire. That isn't likely as the light would be flickering. More likely the button for the contact is touching the pad and grounding out.
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