What is the name of the ABS cap?/nipple? and is it necessary to have?

[Doodle]

Hi and hello!
I noticed that my K75 RT (1990) have a rubber cap?/nipple? on the ABS on one side and not the other. I tried viewing diagrams of the ABS but its not on there and I don't know the right term for it so I can't search for it.
I have tried searching for the number on it, but nothing useful comes up.
Dose any one know the name for it or if its necessary to have on?

[frankenduck]

Brake bleeder cap.

Not 100% required but its purpose is to keep contaminants out of the bleeder valve.

It's a generic item so you don't necessarily need a BMW one. You do want to get one made for braking systems though to ensure it is resistant to brake fluid.

[Doodle]

Thank you, its so hard so find information when you don't know the right name for things.