The three brake lines from the MC, two go to each of the left and right front brakes. The third goes along the bottom of the chassis to a splitter located somewhere around and above the rear axle. From that splitter through the two lines to each of the rear brakes.

Your bake fluid likely leaked out through a brake line that has corroded, check the under chassis line where it passes through the rubber cushioned brackets, that is the usual spot that these lines corrode. Salt and dirt get in between the line and the surrounding rubber over time and vibration you end up with the line leaking.