Chevrolet Repair: 94 chev 1500 brake problem, power bleeder, chev 1500

Question
I started with a soft brake pedal, then replaced master cylinder with no change, rear brake line rusted & broke afterwards.  replace brake line, bled all 4 wheel, no warning light on, pedal pumps up and gets hard but when motor runs brake pedal goes to floor.  brakes are RWAL.  any advice greatly appreciated!

Answer
Hey Donnie,

 First thing to start with is making sure the rear brake adjustment is ok.  
 One of the most difficult things to correct is a GM proportioning valve that has shot over to one side and won't return.  IT gives you a hard pedal even though there is still air in the rear line and when you start it up...... right to the floor.
  The proportioning valve is based on hydraulic pressure over spring pressure.  There is a valve in that is supposed to center on two springs and when a line blows the differential of pressure shoots it over to the low side to block off brake fluid in an emergency.  Once the repair is done the valve is stuck to the side that you are trying to bleed and makes it difficult to get the air out.
 What you want to do is gravity bleed (open bleeders and just let it run out) of all four wheels for a while.  (5 min will usually suffice).  Then have someone get in it with all of the bleeders closed and open up one of the front caliper bleeders and tell the person inside to hammer it to the floor and hold it.  when they have done that close the bleeder.  Have them watch the brake light (key on) on the dash and see if it goes off or blinks.  If it blinks that means it shot over to the other side and now you can get the rear air out.  When you are bleeding the rears this time do it VERY gently and don't go all the way to the bottom.  About 1/2 is perfect travel but very lightly when pumping up.  This will make sure the valve doesn't jump back.
 Before I bought a power bleeder I used to have to do this all the time on those trucks.  Now that I hve a pressure bleeder I don't usually hve to unless someone has been driving around for a while and the valve is too stuck.
 Give it a try and see what happens..

Wayne