Brake fluid reservoir rubber?

[650]

Evenin'

My brake fluid reservoir has the little rubber insert in it, I've seen quite a few other bikes without it. Is it an important thing to keep?

Also, I'm going to purchase a new chain and sprocket set, what T combination will be best for economy?

Thanks!

[Bibio]

the little rubber insert is there to stop air getting in as the level drops the insert expands to fill the void. without it the fluid would go off quicker as its open to the outside air which is full of moisture.

[monkey]

As well as possibly absorbing moisture from any air ingress (which could cause spongy brakes) it may leak brake fluid.