I have a 2002 F250, the auto hubs will not operate. I have done all the vacuum tests and have narrowed it down to the left knuckle. It will not hold vacuum at all. I have switched left to right and right to left locking hubs and still it won't hold vacuum. I have even gone as far as the fitting that goes in to the knuckle. The only thing I can't figure out how to check is the knuckle. The rest of the system is all fine with no leaks up to that point. Any suggestions? The locks work fine manually. It's not the end of the world but as I have gotten older and the price of fuel goes up I have grown a little fond of the automatic locks.

 

Sounds like the large inner seal is leaking.

If swapping the hubs, and the yellow o-rings are good should only leave the seal..

 

Thanks, I didn't know there was another one in there, I'll give that a try next weekend.