My 99 Ford Expedition is experiencing braking issues. Periodically, not all the time, when pressing the brakes to stop, the brakes become very stiff and there is a hard vibration (that feels similar to a drawbridge being lowered) which causes a decrease in stopping power. A couple of times now, I have felt like I'm not going to stop in time. What could it be? It's been to a mechanic a couple of times. An ABS sensor has been replaced, parts of the brake line have been repaired, but no real improvement in brake performance...

The symptoms you are describing sound like the brake system is going into antilock mode. If you are driving under wet or slick conditions, this is normal operation and it may not "feel" like it to you, but you are actually stopping in the shortest distance possible for the conditions.

On the other hand...If it is doing this on dry pavement and you are not driving like a maniac, this would be what is commonly called "FALSE antilock mode". This happens when the antilock brake computer "thinks" that one of the wheels is locking up when it is very apparent to the vehicle operator that there is no way a wheel should be locking up or skidding. This ia almost always caused by a weak or defective wheel speed sensor.

I have had a couple of cases on Ford trucks where the front wheel bearings had been changed with aftermarket bearing assemblies and this is when problem either started, or got worse. Upon inspection of the vehicle I found that these new bearing assemblies came with a new wheel speed sensor already installed. For some reason, the manufacturer installed the sensor with a spacer under it??? (like a metal gasket) Removing the sensor and discarding the spacer fixed the vehicles.