Earlier today I did a changing of my brake fluid as suggested. It was very dark brown. After changing it everything turned out great, however I have one concern. During the process of taking the cap to the reservoir off and on while bleeding the brake system, I noticed the black circular rubber seal that's usually retained in the top of the cap itself would get seated and stuck in the fill hole of the reservoir. I would have to proceed to pick at it and pull it out from its edges. It would be seated in there very flush and even though. So my question is, is this normal and if so why didn't it do it before? What is the correct operation of this? I've only removed the cap maybe 2 or 3 times prior to this and don't recall this ever happening. The rubber seal would stay stuck in the cap itself .