I've never owned an Ampico A, but have built a set of intensity and
crescendo units for evaluation.  The crescendo system works by having a
variable orifice to feed air at a controlled rate to two separate
bellows units.  One bellows is the bellows attached to the crescendo
unit.  The other is the bellows attached to the intensity valves, and
is the spring pneumatic.  Restriction in either of the orifices in the
crescendo will slow the rate of rise of vacuum in the spring pneumatic.

However, leakage to atmosphere, after the orifices in the crescendo
unit, will result in the air flow through the orifices to be split into
two paths: one path will attempt to close the pneumatics, and the other
will flow to atmosphere.  The leakage to atmosphere will reduce the
maximum available vacuum level available after the orifice.  The
leakage will also reduce the rate of collapse of the pneumatics.  With
the crescendo operating on fast speed, the available airflow is higher
so the maximum vacuum level will be closer to the 20" (30") available
from the vacuum supply.

To check where the leak is, disconnect the spring pneumatic from the
crescendo and block the connection.  Operate the crescendo, and it
should be possible to produce full vacuum inside the crescendo
pneumatic.  If not, the crescendo is leaking.  Then repeat the exercise
with the spring pneumatic.

Paul Rumpf