Hi All,  As two authors noted yesterday, the 'shimmy' problem is an
imbalance in the pump.  To verify this, you must first disable the
spill valve by inserting a small nail in the spill valve spring which
prevents the valve from pulling open under tension.  Then connect
a 'Tee' connector to the to the supply fitting.  One leg of the 'Tee'
gets connected to a vacuum gauge and the other leg gets connected to
an open hose with about a 3/16" leak.  That 'safety' leak will prevent
the pump from locking up or bogging down.  Now run the pump.  You will
clearly see the imbalance in the movement of the gauge.

Most likely, as another author said, the problem is that one or more of
the bellows is creating more (or less) vacuum than the others.  However,
anything that creates more or less resistance in one or more of the
bellows will cause the pump to vibrate -- like a cylinder in a car
engine that has a bad valve, bad rings, bad crank bearing, etc., etc.

Musically,
John A Tuttle
Player-Care.com
Brick, New Jersey, USA