I'm backing down a hill today with a heavy load ('87 F250)when suddenly the brake pedal is to the floor. On with the emergency brake, and I get out to the sound of a stream of brake fluid coming from the broken line that goes to the driver's side front disc brake (the metal line broke right before it goes to the flexible line that runs to the caliper). My confusion is this: I thought that "dual braking systems," such as the one on my truck, were supposed to leave you with brakes on two wheels when a line somewhere breaks. Is that not true? If it is true, why did I lose all braking?

Thanks