If I'm thinking of the right piece, the trumpet is a flow control valve if you want to tone back the flow. There's also a venturi in it if you want to use that.
The removable cap is how you prime the filter, use a funnel to fill up the tube before you hook up the quick disconnect lever to the canister.
Then, when you hook up to the canister it will fill the can and the tubing, getting you a good prime.
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