brake issue 80 Blazer

[dubie]

The brake light has been on since we brought this truck home. The previous owner had a rear brake line blow out on him so we replaced it along with a bad front line and the master cylinder, then bled the system out. The pedal still has weak pressure and the pedal won't engage the brakes until 4" from the bottom. Would that rear brake line blow out cause and issue in the proportioning valve? If I slam on the brakes, the front ones lock up while the rear don't seem to engage. There was fluid coming out of the rear bleeders but not as much as there was up front.

Any ideas boys?