'98 Econoline brake booster?

We had a customer bring his '98 conversion van (E150, ABS), and he had a master cylinder he wanted us to put on. We changed it out and bled the brakes extensively but the pedal would
still go way down without any resistance. Thinking the master cyl was defective, we ordered another one, and the next day put it on, and same exact deal--no pedal. There was never any sign of leakage anywhere. It was later decided that it must be the booster. But I don't understand how a defective booster would cause a vehicle to just act like the brakes have air in the lines. I would think it would, rather, just have hard pedal with no assist, as when the vehicle isn't running.
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Did you verify that the problem was in the master cylinder to begin with, and not another problem, such as bad flex hose, rear linings worn, wheel cylinder leaking, RABS valve leaking internally, blah blah blah? Did you bench bleed the master cylinder before installing it? and yes, a bad booster can cause a spongy pedal.
BTW this is why shops charge for diagnosis time, even when the car owner is sure what to fix. It protects both you and the car owner from exactly what you have here.

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Yes, we bench bled the cylinder before installation. As I said before, there was no evidence of leaking. I guess my question was, how does one go about diagnosing it to be a bad booster when such problem will perfectly replicate the symptoms of a bad master cylinder, which is way more common?
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rear drums over sized? Are the calipers binding to one side? Are the hoses ballooning? How thick are the linings?
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Gomer Einstein wrote:

couldlock up all 4 wheels with my pinkie finger... took the booster apart and found a spring which controls the amount of assist to be rusted away..... made my brakes feel very spongy allthough they were fine... the boosterwas just way over boosted and the "spongy" feeling was the brake hoses stretching. i replaced my booster when mine started having a vacuum leak and my brake felt great with no changes. (changed the booster and didnt bleed my brakes,,, never had the master disconnected)
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