how slowly do you changes your gears?
it should be done in 0,5sec

i noticed on my cossi
if i close the throttle very slowly-its not
engouh vacuum to open the dumpvalve .
it opens 0,5sec later-you can hear it chatter.

you have to close the throttle fast to get
vacuum on your dump valve.
how thick is your hose diameter?

i use a baileys 2pistons valve.