That's what happens. As the leak grows the pump builds less pressure and you have less and less power assist. Once it fails completely you'll have to stand on the pedal to have anything happen. Some folks describe that as 'loss of brakes' because they just can't stomp hard enough. If you're a good sized guy you can probably generate enough force to get it to stop.
A good parking brake, using engine braking when appropriate down the hill, and travelling slowly might be prudent.

 

Dude,no update?