2012 Ram 1500 brakes went to floor

Sitting still, pumping the brakes it's fine. Engine running and pumping, feels fine. Hit and hold, holds a firm pedal. Light touch and hold, it's firm. Pull it into drive, ease off so the truck will roll a foot, then reapply, goes right to the floor. Is this an ABS thing? Why is it fine and then roll it forward a touch and it's going to the floor? Fluid level a touch low but not out of the norm? Front brakes are worn but 3/16" left. They aren't on the backing plates. Will get them replaced this week. But why does the pedal dive when you go forward even a foot?

No ideas huh?

Replace the brakes and get back to us.

You have to start first with replacing the worn parts..3/16 is low..pistons are almost out of the caliper..if fronts are low then most likely the rears are low also. .did the dash brake light come on when the pedal went low?

no dash lights came on.

If there are no faults being reported, it sounds like a possible half-busted booster or check valve.

internal leak in the master cylinder.

If a booster goes bad the pedal gets rock hard not to the floor.Its either because of low pads etc or a master going bad.
Rocky