Looking at the engine from the front..Top left of the engine block, it is recessed. There is a black button on the top of it, if you press it, your parking brake light will come on, inside the car on the dash.
It is a closed system, if the fluid has dropped, it means your pads have worn, else you have a bigger problem.
Bleed your brakes, if it hasn't been done for a while, it will give 100% improvement. (its worth buying 1L and renewing it all)