No brake lights when either blinker is on

[aggiegrad]

My trucks brake lights have always worked (or so I thought). I have had several people following tell me my brake lights dont work but they were always fine when I test them. It has passed the yearly inspection at least 3 times with no problem. I have finally come to realize that the brake lights only work when neither turn signal is on. The turn signals work fine but just keep the brake lights from lighting also. Anybody run into something similar to help track down the issue?

thanks for the help and knowledge.