The cap that fits onto the master cylinder. These are removed and fitted as required for your master, and it is very important that all parts of it seal correctly otherwise fluid will leak from it.

A - the brass fitting.
B - the brass nut that secures the fitting.
C - the red fibre washer (there is another one on top) that seals the fitting to the cap.
D - the rubber seal that seals the cap to the master cylinder.
E - the plastic tube from the EeziBleed reservoir.
F - the tapered alloy cylinder inside the end of the tube that wedges the tube into the brass fitting and forms the seal between tube and fitting.