Power brake booster

I posted this on Ford and nobody really answered. Had a '98 Econoline 150 with ABS which had the brake pedal go pretty much all the way down--with no sign of leaks, and fluid in the
reservoir. The owner had a master cylinder he wanted installed. We did this and bled the brakes extensively (first by pushing on the pedal and afterward with a power bleeder) but the pedal remained similarly soft. Thinking that cylinder must have been defective we got another and did the same procedure, with the same result. Last I heard it was determined the booster was the problem. It doesn't seem like the booster and master cyl would exhibit the same behaviour in the pedal if defective, but I've heard they can. My question: How can it be determined that the booster, rather than the master cylinder, is the culprit--if these two problems exhibit the same characteristics in the pedal? Or DO they?
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FYI. If you have a problem with a newer Caravan ABS brake system that requires bleeding of the Hydraulic system you will not be able to get the air out of the system unless you have a scan tool to perform the Bleed function. You can put 20 master cylinders on the van and still have problems. Im not a Ford tech but this may be something you want to look into
Glenn Beasley Chrysler Tech
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FYI. If you have a problem with a newer Caravan ABS brake system that requires bleeding of the Hydraulic system you will not be able to get the air out of the system unless you have a scan tool to perform the Bleed function. You can put 20 master cylinders on the van and still have problems. I'm not a Ford tech but this may be something you want to look into
Glenn Beasley Chrysler Tech
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