What's the rubber thing for?

[Doug Rasmussen <DougRasmussen@domain.elided>]

Exactly what is that rubber thing under the cover plate on the master
cylinder for?

Here's the situation, I'm running manual front discs with the stock 800 MC.
 The MC was designed for drums so the front reservoir isn't as large as it
should be for discs.  As the pads wear the system draws the fluid level
down fairly quickly.  For the second time in 3 years, I lost most of my
pedal while driving.  This time I caught it soon enough so I didn't pump
air into the system.  A quick stop and  refill and my full pedal was back
again.  Last time, I pumped air into the system which meant the fronts had
to be bled and that's a pain.  (Note, my system is not leaking.)

I'm planning on machining an extension spacer for the top of the MC to
increase capacity.  And I'm wondering the function and how important that
rubber thingie is?  I don't think my Toyota's have one.

Thanks,

Doug