Top up hydraulic brake fluid? So I've just gotten a new bike which has disk brakes, and it's my first time using disk brakes. One of the brake pads is rubbing a little on the rotor, so I decided to turn the barrel on the lever anti-clockwise to "take some tension off the cable" (bear with me). 

When I pulled the lever and fluid shot out I became aware of 2 things:


there is no "cable" because my brakes are hydraulic.
I'm an idiot.


The brake still works but now that there's fluid missing do I need to top it up?

(I also need to learn about hydraulic brakes).
Your brake is a closed system, and your brake cable is filled with a mixture of brake fluid and air. Air is compressible, while brake fluid isn't. So, the larger the proportion of air in the system, the more compressible it will be, the more effort is required to actually stop the bike, and this is frequently described as "spongier".

Now, in your specific case, you need to judge whether you have enough stopping power for the environment/conditions in which you ride. If the answer is "No", then topping up the reservoir (i.e. replacing air with brake fluid) might make sufficient difference.