[Closed] Disc brakes

Pulling the lever compresses any bubbles in the fluid, making them smaller and more likely to travel back up to the reservoir, it'll work just as well if you leave the cap on as long as you don't subsequently turn the bike upside down allowing the air back into the hose, look at motorbike or car brakes, the reservoir isn't sealed like mountainbike brakes, it's usualy just a plastic cap with a hole drilled in it to vent!

Probably nothing different to if you took the cap off, held the lever with a zip tie and gently tapped the caliper, hose and reservoir with an allen key. You'd achieve nothing by leaving it overnight that you couldn't achieve in five minutes.

Thanks