Bernt Damm's problem is one that is sometimes encountered on
instruments whose original design was marginal in respect of
self-excited oscillations, and has been restored not exactly in
accordance with that original design.  Minor changes, thought to be
insignificant, can push the design into oscillation.  Here are some
solutions that can be tried.

The spill flap must have a small closure spring, such as that which
Bernt has already fitted in his option (1).  This is simply good
practice and is unlikely to stop the oscillation.

Changing the regulator spring force will affect the working pressure
of the instrument and should not be entertained.

Changing the relative natural frequencies of the spill and regulator is
what is required.  They must be detuned or damped.  I have successfully
used a soft cushion felt underneath the leather seal on the spill flap.

I have also found that oscillation is less likely if the leather
covering of the spill valve is extended beyond the flat sealing area
and glued around the edge onto the thickness of the flap.  I think this
prevents the exposed edge of the leather from flapping against the
fixed board, as the air rushes past, which may be initiating the larger
oscillation.

Bernt has already tried changing the mechanical advantage of the spill
control, in option (4).  By keeping the control screw inside the
regulator bellows he is limited in how far he can go.  Pressure
reservoirs in organs often use an external, sprung closed, spill valve
which employ a long lever the other side of the spill flap pivot, which
comes into contact with a fixed bar and thus open when the reservoir is
full.

With the suction regulator we already have the spill valve on the
outside, held closed by the light spring plus the suction, so it is
easy to design a lever that pulls the flap open when the reservoir
closes.  Try firstly with the lever twice as long as the flap.

Another method I have used successfully is to add friction damping into
the system.  Damping is the first thing an engineer will think of if he
has an oscillating system.  Viscous damping, as used on vehicle
suspension systems, is not really practical on our instruments so use
friction damping instead (Coulomb damping for the engineers out there).

All you need to do is use a small piece of spring steel lightly pushing
against the side of the flap, such that it doesn't stop the flap moving
but imparts a small resistive force opposing any motion.  I have used
this on an electric piano which always oscillated its main reservoir
spill during rewind.

Good luck Bernt, and please tell us how you get on.

Nicholas Simons, GB