4 Wheel Drive/SUVs: 4wd problem, ford f 250, vacuum leaks

Question
2003 Ford F-250 4x4.  The front hubs engage but will only stay engaged for about 2 minutes.  The module has been replaced, but the signal goes away and the vacuum leaks off.  What would cause this?

Answer
Dave , there is a switch on the passenger side front fender about the size of your fist, it has a vaccum connection, and a electrical connection, that is probobly the problem with your truck. We keep 3 or 4 of them in stock at all times if that helps you at all thanks ADAM . Oh and it is held on with 2 8mm bolts.