[supereal]

The vacuum brake system is primitive, at best. We set them up on our Sun machine and find hitting the retard specs is tentative, at best. Most of these distributors have piston pads that are badly worn, usually with a deep groove where they rode the point disk, and pistons that flop around in the chamber permitting a vacuum leak. This results in a constant drag on the point plate, degrading the advance. They were designed to prevent preignition knock with the gas of their day. Now, the fuel is very different, and "spark knock" is very rare. If you can detect difference in performance when the adjuster is backed off, something else is out of whack, such as weak or broken springs in the weight assembly, or a badly worn distributor shaft and bushings. Ford made a great improvement when the 8BA engines came along by using vacuum only, drawn by a port above the throttle plate, instead of raw vacuum from the intake manifold.