Yes I did see the link. Think of it like this, the bleed screw and the feed pipe are both channeled to the center of the cylinder, the bleeder can only let air out up to and below that point. Once the brake shoes and drums are on the upper piston will push out and that will be the pocket of air that that cannot be bled off as the bleed nipple is below the pocket of air.. I am crap at drawings so I cant do you a diagram
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Bob