1993 Honda Accord brake fluid reservoir

The brake fluid reservoir has cracked and broken on top and a small piece of plastic has fallen inside. Does this opaque white plastic cover have to be air tight for the brakes to work correctly? The fluid level in this reservoir is fine and brakes work o.k. but I'm worried about the long term effects if any. The rubber fill cap is o.k. I am talking about the piece on top surrounding it that has the "caution" info on it stamped into the plastic. I have taped it with black hose tape but will replace it if you think it is important.
Thanks.
Jim Bateman
Bandar Seri Begawan, Brunei
Borneo