Terry, I hesitate to suggest something so obvious to one as
technically knowledgable as yourself, BUT, have you checked to see
that the 7T tracker bar tube is unobstructed?  Does this (transition)
action have the funny cancel pneumatic with the extended finger that
opens the 7T and 7B tubes at the expression cutout when the AMPICO
switch is "off".  If so, what happens if you push the finger away from
the cancel tubes while a roll is playing?  If the (treble) intensities
cancel then there is a good chance the cancel signal from the tracker
bar is not making it that far, and the remedy will be obvious.  If it
still fails to cancel, remove the 7T tube at the expression unit
itself.  If still no cancel, you will simply have to demount the
treble expression unit and analyize the operation of the 7T valve
block.  I'm voteing for a tube blockage somewhere along the way,
however.  Sheeshe!  In 8-9 years haven't you used a tracker bar pump?!

<Large, toothy grin!>  -John Grant