Technical Ad Blue “Vacuum”

[rolopolo]

Morning all,
A new issue, a year ago my adblue tank split (breathing issue I was told) was under warranty and they replaced the tank, filler neck and cap

Apparently they were modified to aid breathing (the blue cap was different and looser when fitted)

I’m now noticing a vacuum again,
The cap and neck are clear of any crystallisation, the cap still is “loose”

I release the vacuum every time I stop

But is there another breather somewhere that’s blocked?

Any advice would be appreciated

(High mileage, motorway use, 1 fill per week of ad blue approx 10 ltrs) so it’s not sitting around crystallising

Cheers

Ducato 20 plate, 2.3 180bhp