blackwolf wrote:

Yes, should have an O-ring.

A useful mod I did to mine is to drill a small hole radially through the cap just under the flange (between the lower face of the flange and the O-ring) to allow the air which gets trapped underneath when fitting the cap to escape, and similarly the vacuum created when removing the cap to be relieved. With some lube on the O-ring it becomes easy to fit or remove yet remains suitably snug. The hole is invisible when the cap is fitted.

Before I had this stroke of modest genius my cap would slowly eject itself a few mm after fitting due to the trapped air pressure and was impossible to keep fully inserted.

Just a quick comment on the above ... bought my Optimill system just over a month ago and the cap supplied was pre-drilled with a 1.5mm hole just under the flange so no issues with trapped air

John 2010 Defender 90 300Tdi CW (White/1994)
2021 Defender 90 300Tdi SW (Blue/1994)