The air exit hole in the master is very tiny. Smaller than a typical air bubble. Slight movement of the lever squeezes the air out of the system through that tiny hole, assuming they have all risen to the highest point in the fluid system. This hole also feeds fluid from the reservoir into the system. Lots of air can prevent the fluid ingress. Small lever movements positively get fluid into the system.
I discovered this fact through trial-and-error when I was first learning to bleed the brakes on my 750. Particularly when filling the system from empty, I've developed the habit of pumping up the system with teh resevoir cap off, using very small lever movements ( < 1 inch) while watching the bubbles coming out of that smaller hole. Bubbles coming out means fluid going in, yes? So, I pump up that way until I get the system mostly full, i.e. very few bubbles rising. Refill the reservoir as necessary throughout the remainder of all this, of course. Don't want to suck more are
IN lol.
I leave my brake light switch T adapter unbolted from the fork while filling the brake lines, and I occasionally move/rotate/shake/flick the T adapter to allow any air trapped in it to percolate up towards the MC. I repeat this occasionally throughout the rest of the procedure.
Once I have some pressure in the system, I've also learned to "flick" the lever by holding it down tight for a second, then releasing it suddenly. You'll see a stream of bubbles rise from the hole when you do this. I repeat until I get very few bubbles per flick, then I go for the bleeder screw on the caliper and do a "standard" bleed procedure. I usually only have to crack the bleeder a couple of times to get all the remaining air out.
After the above, I usually go for a cup of coffee and a smoke, then I come back and go back to making small lever movements while observing the master cylinder hole. If I get no air bubbles and the brakes feel firm, I consider myself done for the night. I repeat this check the next morning, too because as TT says just from sitting air will tend to rise to the top of the system.
