Originally Posted by JAJ
When the brake pedal is fully released, there's a duct that opens between the reservoir and the master cylinder. As the pads wear, fluid flows from the reservoir into the master cyliner to keep the system full.
When the pedal moves forward, the first few millimeters of movement close this duct off. If you close the duct before you open up the brake lines, no fluid can flow into, through and out of the master cylinder. Surface tension combined with the slight vacuum that forms keeps the system from draining while you work on it.
Good explanation, thanks.