Help me diagnose brake problem

[dieselbegreat]

Please help me diagnose a sudden brake problem. I was braking going into a turn when I felt the pedal "pop" along with a sudden drop almost to the floor. The pedal now has about 3 inches of nothing (no resistance at all) and then a very low firm pedal. The vehicle stops but I suspect that I am now on some kind of reserve capacity in a faulty master cylinder or booster. This vehicle has the JD7 brakes with the hydraulic booster. Also, how big a job is it to pull off the old MC and replace it, besides making sure the system is bled? Thanks.

[dieselbegreat]

Please disregard this topic. The problem is a rusted brake line running to the rear. Admin, please delete this topic. I'd do it myself but it won't let me. Thanks.