Here is my answer to the role of the pallet valve in the pedal-only 
Duo-Art system.

My London-built instrument has a pallet valve mounted on the expression 
box just above the Duo-Art accordion pneumatic.  When the dynamic level 
reaches 11 or thereabouts the pallet valve is pushed open by the regu- 
lator arm.  This lets air into a pair of tubes which are teed into the 
tubes running from the tracker bar to the Bass and Treble theme primary 
valves.

In other words, the pallet valve ensures that above a certain accompa- 
niment level, all notes are played at the Theme level, which is set by 
the suction supply provided by the pianolist via the foot pedals. Pre- 
sumably this was meant to protect pianolists from having to produce 
massive suction levels to provide a theme level audible above a loud 
accompaniment.  But losing any possibility of themeing can't really 
help the music!

Julian Dyer