That peg on the res is only there so you can unscrew the cap...or tighten it.
IF that rubber grommet was new & flexible, you could pry the res up and pop it out. I'll bet there is a slight bead at the end of the part that fits into the grommet. Some furrin' cars have master cyl remote fittings just like that. Old vw for example. They are tough to get out if the rubber is old & stiff.