I'm not a Duo-Art kind of guy, but I would like to offer the opinion that
the bleeds are clogged, not allowing the valves to seat quickly.  The
valve travel could also be too short.  The muffler felt could also be
wrong, not allowing enough air to pass back into the pneumatic.

Don Teach
Shreveport Music Co
1610 E. Bert Kouns
Shreveport,  LA  71105