Bernt Damm was also asking about what his spill pressure should be for
a Duo-Art at zero intensity.
Frankly, I don't have one presently, or I'd take a measurement for him.
I've never had to measure it though, because it is fixed by the speed
of the pump and the muffler felt hole.
Given the pump speed is correct (won't sound good if it isn't), there
will be only two things affecting the pump pressure-- stack leakage (or
other leakage in the system) and the leakage through the muffler felt
It's very very important to arrange the muffler felt so that your
resistance is exactly right. Otherwise, the D/A will never be linear.
The felt must be 1/4" thick muffler felt and tapered, so that you are
able to wrap it around a 1/4" drill and stuff it without compression
into its hole in the side of the expression box.
To adjust the spill valve just behind the muffler felt, bench the
expression box and on the test stand, draw down the #2 and #8
accordions. Pull the muffler felt out, place a small "Post-it" (3M)
note about 2/3rds of the way over the hole. Now adjust the spill foot
on the Accompaniment side of the box so that you see the note paper
just spring away from the hole due to its own natural curl as you raise
the knife valve. The spill valve is now set on that side for a #10 int.
Tighten the foot and recheck. Then adjust the foot on the Theme side in
the same way. Make sure they can't slip back.
I frankly doubt, from your description, that your problem is "too much"
pump pressure. These pumps are all positive displacement, so their
unloaded pressure through a fixed orifice will be all the same, unlike
the vagaries of centrifugal pumps, whose variations of even 2% could
mean disaster for an expression system like the Duo-Art.