If its got that low then theres air in the system.......so you've found the reason the brakes dont work.

HOWEVER......with standard brakes even worn on every wheel to the limit, the fluid should never be able to go below minimum, so, i expect you have a wheel cylinder leaking slightly.

You need to check for leaks, particularly the back, and if/when its fixed, DON'T fill the fluid to maximum, because when you fit new pads it will all over flow out the reservoir all over the engine bay when you push pistons back etc.