Just do a self check on your brake fluid reservoir first, even its low and it does not mean your vehicle has a leakage. As CeLL said, It could be low on brake pad. You can also try pump your brake pedal several times and hold it when the car is not starting and with key off the ignition. If you have pressure on brake pedal, and I am pretty sure you have no leakage on your brake system. If you really concern about your safety, drop by I take a quick look and I will give you some idea what's going on. If you come by tomorrow, better after 12 because I am having a fun fest on tires in the morning.