Lights that shouldn't be on

Every now and then the dash and front turn signals will turn on when I hit the brakes on my '87 GMC.
I've noticed the front turn signals on other '70's-'80's GM cars and trucks turn on when their owners hit the brakes, but never knew the cause and had never experienced this "phenomenon" myself. And I, naturally, had no idea about the dash lights.
Has anyone had this problem before and how is it fixed?

I have seen that except the dash lights, was a signal switch that usually fixes it. when you say dash lights do you mean just the indicators for the signals? if so, yes that sounds like the signal switch