Hiya I think the reason they bled when left over night is you probably do have an air lock but had built up some pressure behind it, over night it crept fluid past the blockage, We used too do the same sort of thing on motorcycles that where hard too bleed, Pull the lever all the way in tape it too the handle bar and leave it over night then in the morning cut the tape and release the lever slow, what that does is if there is any air the pressure you have trapped will force it slowly too the brake cylinder where it can be bled out, or it will force fluid by it so it will be sucke back too the master cylinder. I have done the same on my Chevy with a piece of broom handle cut too length, just shove the pedal as far and as hard as it will go wedge it down with the broom handle over night then in the morning, with your foot on the pedal take out the broom handle and slowly let the pedal rise, The pressure over night will force any air sometimes past the wheel cylinder seals over a period of time. but the fluid is too thick, in a good system with good integrity. This really does work supprisingly enough.
