Compensating Euph valve alignment -upstroke

[NotABaritone]

Can someone clarify me the pathway of air on an open note On a compensating euph? I am concluding that air goes through the 4th valve because the main slide is directly attached.

I want to know what I should see on the upstroke of the 4th valve when I look down the main slide opening. My down stroke is aligned properly and the holes line up, but I can't determine the size of felt based on the upstroke.

On most horns I've seen, if the felts are the same on all 4. Valves, there is a partial obstruction by the fourth piston when it is up.

[NotABaritone]

Thanks , as usual Mr. Bloke