Hi Chris,  The problem sounds relatively simple.  The level of vacuum
is so high that it is collapsing the air-motor governor to 100%, thus
cutting off all but a slight amount of vacuum to the air-motor.

As with most machines, player pianos are designed to operate within
certain limits.  With regards to vacuum levels, the minimum required
level is normally around 5 inches of water vacuum.  The maximum level
is typically no higher than 30 inches in an upright player piano.

If the applied vacuum level exceeds 30 inches, the system cannot be
expected to operate normally.  Your posting indicates that your Cable
Euphona has been electrified; in other words, an electric vacuum pump
has been installed.  The obvious question here is: What is the vacuum
level of the pump?  If it's 40 inches or more, the air-motor governor
will not work correctly.  However, the "fix" you have already instituted
will allow you to continue using the player system until the excessive
vacuum level damages other sensitive components within the piano.

For the reason stated above, I would first suggest that you install
some sort of a voltage (or current) regulating device to reduce the
vacuum level to an acceptable amount.  A simple [lamp] dimmer control
will do the trick for the time being.  Next, I would strongly recommend
that you start locating and repairing (or rebuilding) the component or
components that are leaking so badly that an excessive amount of vacuum
is required to get the system to play music.

(I have been in private contact with Chris via email since Jan 9th,
2005.  In my first response, I suggested that he try hooking the
system up to a "low-power vacuum cleaner".)

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