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?