If everything seems okay up front (brakes, steering & suspension), look at the rear end - as jt1200r suggests.
From GM's brake "Fundamentals and Basics" for their service techs. I deleted everything you have already covered:
Brake Pull
REAR DRUM BRAKES
Causes
•Right and left side brakes not evenly adjusted.
•Brake linings contaminated with brake fluid, gear oil or wheel bearing grease.
•Uneven tire pressures or tire size. Tire pressure and width directly affect the braking ability of each wheel. Uneven pressures and widths will result in uneven braking.
•Wheel cylinder seized. A faulty wheel-cylinder can cause uneven braking by failure to apply or to release pressure.
•Brake hose restricted or pinched.
What to Check For:
•Verify that all four tires are the same size and that they are properly inflated.
•Inspect all brake lines leading from the master-cylinder to the calipers for evidence of pinching or deterioration.
•Back off the brake shoe adjustment on each side and readjust. After the brake adjustment has been made, firmly depress the brake pedal several times to seat and center the shoes, then recheck the adjustment.
•Inspect the wheel cylinders for signs of leakage or binding, replace or rebuild as necessary.
•Inspect the axle seals for signs of leakage. Gear oil leaking from the axle seal will usually be indicated by a spray pattern starting at the center of the drum and spreading outwards. If any signs of gear oil leakage are found, the shoes and seal should be replaced.
NOTE: Leakage at the axle seals is often caused by worn wheel bearings. If the vehicle has high mileage it is recommended to replace the wheel bearing along with the axle seal.
From me:
When you bleed your front brakes through the calipers; do you get a good, unrestricted flow that is about equal on both sides as the brake pedal is depressed?
Reason I ask is - you mentioned that the brake "lines" have been replaced. Does that include the steel tubing from the P/C valve out to the rubber hoses - or just the hoses?
