Left turn signals stop when braking

[Big Gus]

My 77 M880 has an electrical issue. All lights & signals work but when I hit the brakes and the brake lights come on, the left turn signal (front & back) stop blinking.
Any ideas?
Thanks

[Gordon Maney]

Closely examine your dual filament bulbs to make sure one doesn't have a filament broken and flopped over onto the other one.

Considering you have an electromechanical flasher, I am pondering just what could cause that.

[Ron in Indiana]

Time to check for a good ground at the lights. What Gordon said also. 77-78's had a lot of electrical issues. My Dad's 77 had wiring gemlins we never could find.

[DavidGB]

Wiring gremlins

If your truck is set up for towing, with a trunk connector for the trailer, it is possible for a fault or faults in that to cause symptoms similar to that which you describe. There are many other possibilities of course (sadly.)
DavidGB

[rxinhed]

I replaced the brake light switch on my 77 W300 and solved that issue about 14 years ago.

[greg rider]

It could also be a bad turn signal switch.

[74w300uteline]

blackout lights.

does the truck still have the blackout light selector in the cab? the blackout setup is a whole different kettle of fish.