Q: Master cylinder missing a wire

Hello. The brake light can come on for a number of reasons. If the brake pedal feels fine, then the problem is usually electrical. If the only light that is on is the brake light it is typically caused by a bad parking brake switch. If this switch fails then the light will remain on as the instrument cluster will get the signal even when the brake is released. A mechanic would typically diagnose this by disconnecting the switch to see if the light goes off. If it does then it is the switch. If not then the problem may be a brake pressure switch. As that point a mechanic would scan the electronic brake control module to see which codes are coming up to determine what the problem is. If this is something that you are planning on having done, consider YourMechanic, as a certified mechanic can come to your location to diagnose your brake light issue.