Nice job on the swap! There is something about those old 80-86 trucks that are applealing.
On the brake light, as stated low fluid in the master, low vaccume from the pump, and E-brake will trigger the light. On the 85 I had to bleed the valve (proportion?) that is located under the steering column mounted in the frame rail. There was a switch that will trigger the light if there is mor fluid going to the front or rear.