If the sustaining pedal releases too slowly, I would guess that the
valve travel is limited and does not open enough to allow atmosphere
to come in.

I am not familiar with the Welte valves specifically but if you have
a valve that is loose -- for example, the top seat which seals it to
atmosphere can be stuck to the valve leather, it barely lets air in to
refill the pneumatic -- it's a situation I can envision.

The three possible scenarios that I can see are

1. that the linkage does not travel freely.  You reported that it does,
but double check this just the same, perhaps by disconnecting the
pneumatic from the linkage and allowing it to fall back in place.

2. the dampers themselves are non-responsive.  Highly doubt that this
is the case.

3. the valve does not allow air to return.  Check the valve, the seats
and that everything travels freely and completely.

Good luck, I hope this helps.

Eli Shahar
Tel Aviv, Israel

 [ Also verify the pouch bleed isn't clogged.  -- Robbie