I use the forum regularly for info so I thought I would make a contribution.

Just rebuilt rear calipers on a 1987 560SL. I drained the most of fluid from the reserve chamber and refilled it as I bled the rear brakes. I could not get the air out and could not figure why. I thought my rebuid was leaking air past the new seals.

Turns out when I refilled the master cylinder I did not notice the rear chamber did not fill. You can't see it well from the drivers side of the car because its only a small chamber on the rear right side of the chamber. From the left side and from the top it looks like its full. The rear chamber only fills when you over fill the main chamber.

Took me hours of head scratching before I saw it.
Phreon00