I installed a new master cylinder and afterwards my brake lights would not work. The tail lights and turn signals still work. Istarted troubleshooting with changing the fuses, changed the brake light switch, and changed the turn signal switch.Since my underdash wiring was a mess, I cleaned that up by installing a new underdash harness also. I still have no brake lights. I also checked the grounds andbulbs. What else do I need to check? They worked fine before the new master cylinder. Also, my back up lights no longer work.

 

Remove the connector from the brake switch and jumper the contacts, a paper clip works. If the brake lightswork it's the switch. To see if you got power to that point, get a 12V test lamp and probe the connector. Being you also did something under the dash, get a wiring diagram.
Jim

 

I will try to jumper the contacts. I have a diagram and I have tried to brake switches so far.

Thanks

 

I checked the contacts and the brake lights worked. The brake switch was bad but something is causing the accy fuse and the brake switch to go when I apply the brakes. I went back through all the wiring using the diagrams and schematics and can't find what is wrong. Should there be any insulation on the portion of the brake pedal that the switch fits on? I didn't have anything on there originally and it worked...just wondering. I think I am taking it to the shop this week if I don't figure it out tonight.