How do you check the brake light switch?......running light works but brake light does not work......bulb is good on both filaments
Thanks
Ed

 

Climb under the truck and check the two wires attached to the brake light switch screwed into the back of the master cylinder with an electrical meter. One of the two wires should be hot at all times (even with the ignition turned off). If that's the case, you just need to buy a new brake light switch. You can probably screw it out of the MC and get a replacement at the local parts store. They're pretty universal.

 

The switch is a very common one, and dosen't care if the application is 6v or 12v or the polarity of the ground.



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And these switches are problematic. They are known to go bad at least three different ways
1. They fail to come on. Very easy to troubleshoot.
2. They stick on, usually obvious when the battery runs down because no one notices the brake lights are on.
3. They fail to come on unless a lot of pressure is put on the brake system like during a panic stop. This is by far the most dangerous problem. I saw cars pass through the Florida State Inspection with this problem. You want the brake lights to come on during an easy rolling stop so as to signal someone following that you are slowing down. This reason number three prompted me to write a safety article for the Thunderbird Scoop back in either the 1970s or 1980s.
These switches are cheap and easy to replace.
Thanks for reading. The rear end you save could be your truck's.

 

Thank you, all is well again with the brake light
Ed
near Philadelphia