I had a similar problem on my 71 C10 I found I had a bad hose on the right front and a cloged line to the rear brakes. Beleave me I thought I was going nuts.In desperation I pulled the brake line off one side in the rear and hit the pedal. After all what could it hurt. NO FLUID so I pulled off the other side no fluid there either. The line from the MC to the rear was blocked I dont know where or how but after changing it I had brakes. I had a firm pedal all the time but no brakes left front was the only thing working. Good luck
Charlie