I have a 72 F250. When I put the left turn signal on and apply the brakes, the signal stops blinking. I checked the bulb and it was fine.

What is the likely cause?

 

Bad ground at the rear left bulb. Check the outer brass part of the bulb, and the little tab that contacts it, make sure they are clean. Then check the ground wire leaving the socket from this little tab, I have had the socket go bad internally. And then follow this ground wire, checking it along the way for bad connections till it gets to where it's grounded.

If you want to run a test to prove it's the ground, take a scrap piece of wire, put it on the neg battery post, and run it around the truck and strip the end bare. Jamb it down beside the bulb in the socket, and the problem should go away.

 

Thank you Franklin. I am really bad with electrical and of course the truck I buy has many gremlins in this area!