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Brake Bleeding

Do I have a bad Master Cylinder?
I've been passing the time bleeding brakes with my newly fabricated
handy-dandy pressure bleeder, consisting of a reservoir cap with tire valve,
Works great!
Except... I'm getting poor results on the 89 100 (non-quattro, auto trans).
The pedal still feels spongy and I think there is still air in the system.
I just finished doing the 4KSQ with good results, so I'm thinking the
problem isn't simply operator error.
My book says "failure to bleed satisfactorily after a reasonable repetition
of the bleeding operations may be due to worn master cylinder seals".
What are the symptoms of a bad master cylinder?  Would it let air enter the
Is there a way to tell for sure if it's bad?
FWIW, the book says the MC can't be repaired but must be replaced.
TIA for any help.
Dave C.
Dave Conner, Columbus OH   
 '87 5KS, '89 100, 86 4KCSQ