Why dose my turn signal stop working when I apply the brakes 1994 chev

Sounds like the third brake light is shorted to the left brake/turn light. Try removing the bulb in the third brake light and see what effect that has. If your directional lights and brake lights work normally then, I think that short is the problem. The short could occur in the switching system itself or in the wiring to the lamps. Sometimes these circuits are load sensitive, meaning if a circuit is intended to operate one lamp and it tries to do more or less, it probably will produce some odd symptoms. The switching occurs at the directional turn signal lever location and your brake switch. I hope this gives you some leads.