I think D. L. Bullock is right in his observations.  I myself have
modified my Kastner Kastonome Autopiano in the same way.  The only
inconvenience is that I can't use my silent-button as the tracking
device will be out of order since there is no vacuum.  The correct
way would be to make an extra valve in this instance to reconnect the
vacuum of the tracker-bar to the main reservoir.

There is another problem with old rolls: the paper has often been
subjected to moisture.  The paper is therefore often not quite planar
and the vacuum in the four tracking-device holes in the tracker bar
cannot hold the paper tight.  Therefore the tracking-device is quite
often not functioning correctly.

Peter Heinesen
www.peter-heinesen.dk