Most likely it's the master cylinder bypassing. Try very lightly applying the brake. It doesn't have to be moving. If you ever so slowly push the pedal, usually a bypassing master cylinder will let the pedal go to the floor.
Another thing to check is the front wheel bearings. If there is a loose bearing, the wobbling wheel will cause the rotor to push the piston back into the caliper. Then when you hit the brakes again the piston has to come out to meet the rotor.
And if it has rear drums, a stuck adjuster will cause low pedal.