If you look in the bottom of the reservoir, you'll see a metal plate with a couple small Philips screws. That's what attaches it to the MC housing. Yes, there is an o-ring around the O.D. of the portion that inserts into the housing (for sealing) and yes, as Jayel said, it comes out easier if you can twist it back and forth. But it's probably worth trying to remove it. That would make removing the broken screw pretty easy. I highly recommend using anti-seize on the new screws so something like this doesn't happen again.