There is only 1 brake fluid reservoir. The other is for the clutch.[
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As a general rule, you should frequently check, but never top up, your brake fluid. When you have new pads, the fluid level should be at the max mark. As the brake pads wear, the level will drop (because some of the fluid will be used to push the pistons that little bit further out). If you top up the fluid at this point, then it will overflow when you next put new pads in.
If your brake fluid level drops for any other reason, it means that you have a leak, so you should get it checked and fixed.
If your brakes feel a bit spongy, it could be that you just need new brake fluid (assuming that it wasn't done when you last had it serviced). Changing brake fluid is actually quite a simple task, which any competent mechanic could do on their own car. However, if you aren't 100% certain about your ability in this area, you should take it to a garage and get them to change it for you. It's one of those jobs that, although simple, really has to be done properly. Otherwise you could kill yourself (or even worse, somebody else).
If you've recently had new brake fluid, then it's going to be something a bit more major in the braking system that will need attention, so you should get it checked out ASAP.
Cheers
James