Hi,  A few assumptions and a couple calculations suggests that the
maximum vacuum that a tracker bar pump might produce is something
around 320 inches.  With the pump plunger all the way down (zero
volume, not counting what's inside the nose of the pump) the system
volume in the piano (tubing, valve chamber, etc.) is someplace around
0.75 cubic inches.  With the plunger all the way out, the total volume
is about 3 cubic inches (0.75 plus the internal volume of the pump at
around 2.2 cubic inches).

So the best you could ever do is to expand the volume by about 5 times.
That lowers the vacuum from the theoretical 405 inches to about 320
inches.  Add in a little leakage and something from the bleed and
I doubt that you'd ever see 300 inches vacuum, even if you could pull
the plunger instantly.

Regards,
Craig Smith