I rebuilt the rear wheel cylinders in my 1958 6400, then tried to bleed and could never get a good pedal. After I rebuilt tha master cylinder, replaced all the lines (steel and rubber)and rebuilt the front wheel cylinders (pistons in the fronts were frozen) I bled it again and now have good brakes. (home-made power bleeder and engine off). All that said, make sure everything is in good shape or it may suck air in.
Also, make sure you adjust the top shoe on the rear brakes properly for bleeding (loosen it all the way up so it collapses the piston all the way into the wheel cylinder and keeps it from creating a pocket that can trap air.)
