Anyone run the AP racing "race" brake reservoir cap?

[KnifeySpoony]

I have read about the standard cap leaking on track, so I took the internet's advice and got the race cap with bellows. I just want to make sure I've installed it right. The new cap has a white plastic insert and O-ring. My understanding is that these must be removed when the bellows is used. Can anyone confirm? I have fitted it this way and it fits, but I can't tell if the top of the bellows (which now presumably acts as the sealing ring) is seating well within the cap. My goal is to prevent leaks; I don't want to create a massive leak by installing it incorrectly...

 

Thanks