[Crashmaster]

I have no clue as to the specifics of the 404's braking system, but to bleed a master cylinder in the vehicle, you can just crack open the line's flared fitting like bleeders while someone is pressing on the pedal.
However it sounds as though the leak that is allowing your fluid out also lets in plenty of air. I don't know how the master and pressure booster are arranged in your system, but for a leak that is not externally evident, I would suspect something like a worn piston cup that leaks out the rear of the master cylinder(or air assist mechanism) and into the booster. The cups are not designed to hold much vacuum but to seal better under pressure. A small fluid seep could allow alot of air to be pulled in.
Hard to make an accurate guess as I don't really understand how the assist mechanism is plumbed.