Brakes won't hold back wheels

[davepl]

After adding trans fluid I lifted the truck off the ground a couple of inches, fired it up, and put it in drive. It didn't even stall (insert advertisement for Quadrajet and stock setup here).

However, I had to push on the brakes with all my might to stop the rear tires from turning. I'd like to claim its sheer torque but I imagine something's not right.

Note that because it was up in the air you wouldn't normally see this, as your fronts would hold you. I don't know if mine will, and I'm not crashing to find out!

The only different thing about my setup is I converted to '72 3/4 ton front discs. Kept the '70 rear drums. Booster, master, combo valve, everything else is from the '72.

In any event, the pedal feels fine. I'm wondering if it could be as simple as the star wheels not being set right.

When I turn the wheels by hand I think I can hear a light scuffing, so I thought the pads were engaged.

Or should I be turning them until the wheels WON'T turn and then backing off? Not sure how to get the proper adjustment. Dana 60 3/4 ton.