1991 freightliner fld120 Brake light issues

I have a 91 Freightliner fld120 all the lights work except for brake lights. I’ve changed the switches even though i didn’t have any leaks. I’m thinking I’m not getting air to the switch. I changed the switch on top of the manifold and when i push the brake I’m still not getting any brake lights. Has anyone dealt with this horrible design that can maybe point me in the right direction here. I’m lost.

 

If you have power coming from the switch, then its further down the line and could be anywhere. Start with connectors first.

 

I had similar issues with my 2001, I took it to Freightliner because I didn't have time to mess with it. After they worked their magic, sometimes they would work fine, other times nothing. It ended up being a bad ground near the tail lights.

 

Check if there's power supply to the switch and then step on the brakes and see if there's power flowing through the switch. If yes to both, keep going down the wire until you find the issue.

 

Goes through the turn signal switch. Check that out. If you have a watcher wiggle to switch handle.
If that is OK do electrical checks.

 

I figured it out. My goodness. Quite simple really. After messing with them darn stubborn switches under the dash. It was probably this the entire time…. It was full of trash ans air wasn’t getting to the switch. Working fine at the moment. Maybe it’ll last a week or so. Lol.

 

Now. If you same guys know a trick to getting the dash parking brake valve (tractor/trailer valves) out of this rig. It would be greatly appreciated. Lol

 

Take the top cover off. Remove center defrost vent, remove pannel under it. 5,6 screws.

 

Did yours break, like you try to push in a knob and the the whole thing pushes in lol. I had to replace part of the "frame" behind the dash that holds it all in place. Freightliner can still get the replacement pieces.

 

Just leaking real bad. It’s still has the originalwhite one in it. There isn’t much room for movement behind the cover. I’m going to attempt what what James replied above. Maybe that’ll help.