yellow disc on brake fluid reservoir

Hi all

so i've had the Brake Failure check manual warning come on first thing in the morning for the last 2 days (although the brakes seem fine). I thought i'd check the brake fluid but I can't see if it needs any adding but I found on the top next to the cap a yellow disc with a slot in it. I used the key to turn it and it came off. I've puit it back as I have no idea what it's for! I thought it might be a way of seeing what the fluid level is.

As I want to check whether it just needs fluid adding, if it does need some, is it possible for me to just buy sme and top it up?

(i have no idea when it comes to servicing cars - as if you couldn't tell!)

If I need to take it to a garage that might be a problem, as i've just found out I have to go away on business tomorrow and I need the car to get me down south for a couple of days!!

Any suggestions??