Everyonce in awhile my 91 D250 acts like there are no power brakes. The pedal is VERY hard to push down. It only does this every once in awhile and a few seconds after it happens the brakes are fine again. I am starting to get concerned about this. Do you think my vaccuum pump or brake booster could be going out?? If so how do I test to see which one it is?? Im really puzzled because it is such an intermitten problem. Any help would be greatly apreciated.

 

I think that you could check out all the vacuum lines first, and if you have a check valve in the booster, I would look at that first.

 

Im having the same exact issues,so any more ideas would be appreciated ,I have checked my vacume and it seems to be good,so I dont know if its my booster or what.

 

I had the same problem recently. It turned out to be a collapsing vacuum line. It was a tough one to find as the vacuum tests checked out fine, but the hose would pinch itself shut. See if the big hose from the vac. pump to the booster is squishy.