When the front brake line leaked, all the fluid from the reservoir was pushed out of the leak. Eventually there was no fluid in the reservoir and the piston in the master cylinder was empty and could not build pressure to actuate the rear brakes. Any leak in either circuit if left unplugged will allow all the fluid to be pushed out leaving both circuits inoperable. You would still have the mechanical parking brake to slow you down.Forgive my ignorance but I thought that the dual circuit brake master cylinder would prevent the catastrophic failure in both the front and the rear and a failure like you described. What am I missing? This is near and dear to me because @cruisermatt is just finishing fairly extensive brake work on our 93.