Same here, I use a vacuum pump too. Mityvac? brand pistol grip style works nice and it's not expensive. The hubs need to see a presence of vacuum for a brief period of time to both lock and unlock the hubs. If the pump itself can pull 10 inches of vacuum it is adequate. I've seen as high as 17 but low to mid - teens is good for right off the pump. The vacuum solenoid will pull a vacuum for like 30 seconds to lock the hubs and then it releases the vacuum. If there are no leaks and the lockouts are ok they will lock and stay locked regardless of key on or off cycles. To unlock the hubs the solenoid again will pull a vacuum on them for a short period of time and then release the vacuum. At that moment when the vacuum is released, the hubs should be free.
You can test the pump and solenoid operation with the vacuum tool hooked up right off the solenoid and/ or at either hose at each steering knuckle. Test your hoses and connections for leaks and also right at each hub. You really shouldn't detect any drop in vacuum watching the guage for a minute. Common places for leaks besides a obvious cracked hose or pesky leaking hose connection are the vacuum solenoid, large hub seal pressed into steering knuckle, the wheel bearing itself between the fixed/ rotating flange, or the large o ' ring around the wheel bearing where it seats inside the steering knuckle bore.
I apologize if my specs aren't totally right as I'm just going off of memory here. BTW this system all works exactly the same concept for '99-2010 f250-f550 with autohubs.
