Brake trouble shooting help

[bhemingson]

I just bought a 1972 Cheyenne Super. 3/4 ton leaf spring truck. 350 automatic. All stock. The owner said the brakes might need bled. What an understatement. I went to pick up the truck after coming home from a trip. Brake light on. Super hard pedal near the floor. Almost no braking capability. Front master cylinder reservoir near empty. Q1: Is that for the rear brakes? Q2: with that front reservoir dry I probably have air in the lines but how do I reset the proportioning valve as I understand it "clicks" over when brake warning light is triggered.

I added brake fluid and after about 20 minutes driving on back roads to my shop I started to get a little pedal (in my head maybe) and the reservoir still had fluid. What is the most logical place to start to get brakes without just throwing parts and time at it?