If you can't pull the rod out of the master cylinder, the little rubber grommet is doing it's job.... Depending on how your brakes are adjusted etc, the rod would/could slide right out leaving you with no brakes if someone pulled the brake pedal up if the rubber wasn't in there at all, hence the reason the engineers put it there. I put the grommet in the M/C making sure it was lined up properly and then assembled the rest. it snapped together when I pushed the brake pedal. It wouldn't pull out easily later when I had to remove the rod and M/C.