IIRC you can't gravity bleed drums due to check valve.
Regarding fronts. When you apply pressure to the brake pedal. Do the front rotors stop being able to be rotated?
When you take your foot off the brake pedal can you rotate the rotor?
Discs do not release completely they always drag on the rotors a tinny bit.
Normal flexing of the suspension components, wheel bearings etc help push the piston in it's bore a bit. Only needs to be a few thousandths of an inch to allow rotation. Piston seal also helps.