I just rebuild the front end (ball joints, tie rods, bearings, etc)
including the calipers and rotors and put new rear wheel cylinders on
along with about 8 foot worth of rear brake line, rear brake hose and
a new master cylinder.

I cannot get the front brakes to bleed the peddle is very, very soft.

I believe the back brakes are OK.  I removed the front brake line from
the MC and plugged the hole.  When I did this the peddle was rock
solid.  However, with the front line hooked up (along with the back)
the peddle goes to the floor.

I have went through about 3 gallons of brake fluid trying to bleed the
front brakes.  I replaced the bleeder screw just in case those were
the culprit but it didn't make a difference.  I don't see fluid
leaking from anywhere so there is no obvious leek that would be
letting air in.

Any ideas on where I should look to find the problem?

Thanks in advance.