1999 Ford F-150

1999 ranger, 150 000 kms. I have had problems with the PVH hubs disengaging. Its been a long process but Ive narrowed it down to a vacuum loss to the hubs. I have steady vacuum supply to the solenoid thingy under the wheelwell and when I take the vacuum line off from the output of the solenoid thingy I have steady vacuum for about 15 seconds then. Nothing. My question is. What tells the solenoid thingy to pulse. Is there a module somewhere. I know the PVH hubs are not the greatest but mine work great when they are engaged. They are just losing vacuum. And disengaging. PLEASE HELP. If this is answered quickly I willl definitely donate : )
January 9, 2007.

Not to say this is your silver bullet, but 3 times I had the same problem. The metal vacuum tubes at the wheels corrode. Pull off the rubber vacuum tube from the metal one, clean it, and very lightly put a touch of silicone gasket maker thinly around the tube keeping at least 1/4" from the end of the metal tube and re-insert the rubber tube back on. Then try it.