Doug Vensel asked about his tracker mechanism [150913 MMDigest].
In looking at the rear picture of the tracker bellows, I only see one
tube to the unit. Usually a two-sided unit like this has a connection
to each bellows from the tracking ears.  The bellows need to be as
air-tight as possible, as it works by leaking air into one side or
the other.

I see two tubes going into the bottom of the unit, probably to provide
plenty of vacuum to the tracking bellows.  The spring on the bar behind
the roll box is to provide some balance to the spring loaded left side
roll shaft.

David Dewey