Brake lamp on w/ spongy pedal poor stopping?

2002 chev 2500 express 5.7l R. I have been having a problem with my mechanical brakes for about a month now. at times they fade drastically with very poor stopping performance. When this happens the "brake" lamp illuminates. I pump them and eventually I get pedal and brakes back. I figured it was possibly air in the system so I bled it per specs and have the same result (r/r, l/r, r/f, l/f). However when I got to the l/f caliper to bleed it, my nephew was pushing down on the pedal and when the bleeder was opened... nothing! Then, slowly, a trickle after he applied very hard effort to brake pedal. All others bled properly. Have any ideas? im thinking a proportioning valve but Im not too familiar with ABS. If you have an idea I would love to hear it!! Thanks so much for any help all!