Are you lubing the o-ring with oil prior to installing it?

If so, then yes, another cap may be a good investment at this point.

Good luck!

__________________
Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual