My turn signal doesn't work when the brakes are applied. All signals work individually.Anybody have any thoughts on that?

 

Well sounds like ur losing power somewhere low battery? you probably checked that already umm...
your not running a massive stereo by any chance are you? Im kinda brain storming a bit of possible
problems. Also could be Alternator? last thing I could thing of is a possible loose ground wire.

welp I tryed hope It could help some (im no professional)

 

do they blink really fast? if so it is probably the alternator. i just had the same problem.

 

Check the grounds to the brake light assemblies.