Should be "good to go" after r/r for the o-ring. Sounds like there may be a problem with the float. Possibly got stuck, or jammed in the down position when you had it removed?
Not exactly sure how they're constructed internally. Just did this job on my E300D and had no issues after re-installation.... pretty sure it's the same sensor (not positive). No priming or anything of that nature should be required. Maybe take it back out, see if you can hear or feel the float moving by gently shaking the sensor? Possibly spray a little brake kleen or similar in there to remove gunk, crud, etc.? Dunno, just a thought.