21 is a rubber seal which sits on 22, once you have taken out 23, 24 and 25, only that ring stops it falling out, You should be able to pull it out with a tooth pick or push it from the top, 25 is the only thing holding it all together.
It must just slide out , only an o-ring stop it , But maybe is stuck with dry paint like mine , just gently push it from the trigger shaft hole , trying to not damage the o-ring