Been working on a '92 F-350 and have had 3 vacuum go out. Have bought from NAPA, O'Reilly's and online and all 3 have failed. Is there something I'm missing? Anyone else have issues? And is the pump directional? Am I maybe running backward? Any help would be appreciated. Thank you.

 

Were you following a belt routing diagram for the 7.3 ? Replacing the serp belt was quite a challenge on every single one I had to do and they were many..
Different belt if vehicle had A/C too..

If the belt tensioner is original, it could be binding and applying excessive pressure on all the bearings in alt, w/p, idler pulleys, vac pump, etc..

Wise move is to consider replacing the tensioner, while in there.