New guy needs help with brake problem

[gmc-rob]

I bought a 1972 Chevy Cheyene 10 a few months ago and am trying to get it road worthy. When I first got the truck it braked really well (it has power brakes) but the rear brakes locked up first. The truck needed to have a saftey inspection and to pass it needed new rear brakes, so after putting on new drums, shoes, parking brake cables and all the other rear brake springs it now skids the front tires first and it does not seem like the rear brakes are working. I did not replace the wheel cylinders or hoses so the fluide part of the system was not opened so there should not be any air in the system. To make sure I took it to a brake place and they used a power bleeder to bleed the brakes but the rear brakes still do not want to skid, even in gravel when before they would easily lock up on the pavement. I think they are adjusted properly and I backed up and stopped alot to set them up, what am I missing??? I would really appreciate some advice, thanks. ROB