Braking weak at very low speeds

[D13]

1999 3/4 2wd extended cab. Truck has been sitting for a while because idiot who did the front lines didn't do the rears. Pulled the bed, new rear line and axle lines and hoses (hint - do your fuel sender while you are in there). Now, drive along, apply brakes moderately or hard at any speed, stops fine. Will go into antilock on loose gravel. Brakes just fine on dry pavement too.
Problem is at 'parking lot' speeds, like when maneuvering to park in a tight space. Hold foot on pedal, to lightly apply brakes, get a 'flutter' in the pedal and little or no braking. Let off and stomp and it stops instantly.

Two things come to mind. First, there's still air in the system, both rear bleeders broke off and I'm not confident on the bleeding. But the pedal is firm and proper height and all 4 are working.
Second, it is getting false ABS signal. But I would expect that to be there all the time, not just parking lot speeds.

Whatcha think? Don't really want to dump a ton of money in a rusted out 200,000 mile truck. but can't drive or sell it as is.