I'm not sure if these valves are rigid or wobbly.  If they are the
wobbly type you may have too little wobble.

I had a squealing problem with an Autopiano secondary valve chest.
Had to go back in and move the force fit collar to give the valve
enough wobble (motion) to allow it to seat properly against the seat.

Please let us know what you discover so we can have an accurate
resolution in the Archives.

Sam Harris - Player Piano Restorations Illustrated
Greenville, North Carolina
http://www.geocities.com/anneharris1/