Are you on the brakes when you're slowing down? Maybe there's an excess load (high resistance short) in either of the rear turn/stop light circuits that pulls the voltage down too far with the brake lights operating along with the turn signals.

Or it's possible that your generator is not working correctly and your battery is on the way out.

You need to get out your multimeter and monitor your battery voltage at various conditions.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.