Hi,
I kept having this problem with the breather cap on my Turbo Diesel, there was negligeable pressure but the ribbed sealing ring just wasnt holding in place through age. As I was away on greenlaning trip, removed the rubber sealing ring, applied a couple of turns of insulation tape and then reapplied the rubber sealing ring.
This solved the problem and although was only deemed as a temporary repair this was nearly a year ago and havnt bothered to buy new seal or breather.
The cheapest landy repair you will probably ever have!
Buy yourself an easter egg with the money you have saved!!!!
Cheers