> My problem occurs when I try to install the stack in the organ.

Steve,  This is a tough one to answer without actually being there
with the instrument.  One thought that immediately comes to mind is
that even though your vacuum bellows on the 105 may be drawing 27-32",
it may be lacking volume due to leakage or other problems.

Did you rebuild the vacuum pump?  I'm assuming that you tested the
vacuum with all hoses sealed off.   A poorly operating pump could still
produce a high vacuum in inches, but without enough volume, this vacuum
would drop significantly once everything is hooked up.  Sort of like
trying to operate a standard size air paint gun on a little 1/2 hp
pancake air compressor.  Yeah, it puts out 35 PSI, but at what volume?

Another thought is the cutout.  Is it possible that the setting on this
unit is being disturbed when you put the stack in and causing a loss of
vacuum.  

Another possibility could be the base drum controller, although a leak
here from the signal hose, about 1/2" hose, might not cause that much of
a drop in vacuum.  But the hose to that is fairly large (1-1/4-inch?)
and a loose fit could cause problems.

You state that bench testing the stack shows it to be tight and no
leaks.  How powerful and large is the vacuum pump you use on the bench?
If it is significantly more efficient that the one in the 105, it may
be masking your results and possible leaks.

These are just random thoughts, but those are areas I would check.

Jim Quashnock