2000 Express 3500. Brakes sometimes work normal. Sometimes they will grab hard with the pedal up high, but if I press on the pedal harder, it suddenly slips. Meaning the pedal drops about 1.5" then works like normal (normal is how it worked for the first 98,000 miles). When the pedal is high and drops, I can feel and hear some type of mechanical bind or scraping. Yesterday the pedal got high and very sensitive, the van came to a stop easily only pressing the pedal maybe 1". At a red light I pressed on it much harder and could feel and hear that mechanical scraping/binding sound again until the pedal went down half way.

The first thing I did was change the hydro boost. That worked for a week and back to the same symptoms. Changed the hydro boost again, and also the master cylinder. Again good for a week then the same thing again. Did I get 2 faulty hydro boost units?

 

scraping sound suggests a rotating mechanical , not hydraulic problem. Did you remove the wheels and inspect?

 

It will make the scraping sound and feel in the pedal while stopped. Nothing turning.

 

Have you confirmed that none of your calipers or wheel cylinders are seized? Like joe asked, what have you checked and or ruled out?

 

No haven't checked for sticking calipers or wheel cylinders, I never felt it pulling or anything. I suppose I can try to get it to act up in the driveway then jack up each wheel and try to turn it? I had replaced a leaking wheel cylinder about 6 months prior to this problem and after seeing a little rust in the old one and the fluid looked dirty, I bled the whole system til it ran clear. It just doesn't feel as if one caliper/cylinder could make the pedal as high and as hard as it gets. I forget without looking where the proportioning valve is. I'm thinking if it stuck to give full front brakes without the rear, that would certainly give it a hard pedal.
As far as what I've ruled out, I suppose the master cylinder and the power booster. Now I can't find the website where it says a high hard pedal with brakes that grab hard is a sign of the booster.

 

I second the sticking caliper/bound pad/frozen wheel cylinder ideas. I'd work the front calipers/pads first, they are more likely the cause.

 

If it has ABS, there's a bit more to consider.

 

Re the calipers, it did it today in the driveway and I jacked up each front wheel and they turned fine. For the hell of it, with the pedal still in its high/hard mode I put the rear tires on the paved driveway and the fronts in the stone part, put on the brakes very lightly and gave it throttle. Both fronts locked up and skid in the stone. So the fronts aren't sticking on or off. It started raining hard and I was late so I didn't get to check the rears.

 

If there's scraping while the van is not moving, why don't you just get an assistant to press the brake while you follow the sound?

 

That's a good thought, but having someone here while it's acting up seems to be rare. I don't see my girlfriend everyday but I'll keep it in mind. I will have limited time to listen, it will do it for only one stroke. Sometimes just the first inch of the stroke.