1st-Check the rear drums are properly adjusted. You have not responded back that you did that, if they are way out of adjustment (very probable if you rebuilt the rear brakes and put in the adjusters/star wheels fully retracted and never went back and adjusted them) then you would get the symptoms your talking about.
2nd-If the rear brakes still do not grab after adjusting them, then check the wheel cylinders for being siezed up.
3rd-If the wheel cylinders are working, I'd start looking for leaks or at the Master Cylinder. You do NOT have a BRAKE LIGHT, correct? The fluid level stays constants in the Brake Reservouir, correct?
The parking brake is the last thing to adjust and check. If the rear drums are out of adjustment, they will create less braking force, with a low pedal height, the parking brake will be much looser as well. Only after you get the rear drums working right and properly adjusted, do you adjust the parking brake cable.
Its possible to mess up the rear drum adjustment by having the parking brake cable too tight. If you suspect that, then loosen the parking brake cable, get the rear drums working right and then go back and tighten the parking brake cable.