Voyager problem--need super sleuth

98 Voyager minivan 3.0L 6 cyl  auto trans(4 speed) no ABS  75000mi

Bought this van new in 98--truly a Sunday go to meeting car.   About 1 1/2
yrs. ago started noticing a vibration in brake pedal upon normal braking
from speed (60 mph + up).  Put on new pads, rotors, calipers.  No better.
Did rear brakes--no better.  The vibration has been getting worse as time
goes on.  Took it to a mechanic I trust.  Over the past two weeks he has put
on new front brakes complete with calipers, pads and rotors; new rear brakes
with wheel cylinders and drums; new master cylinder; new shocks; new struts;
new tires; front end alignment; new ball joints; The problem is no better.
Vibration is only felt in the brake pedal--steering wheel is fine.  Car has
not been in any collisions.  ANY HELP APPRECIATED!