Ford vacuum leak in 4x4

my ford’s 4x4 will work if I manually lock in the hubs. If i use the auto option it will not engage. I can feel the rear end diff lock when I put it in 4-low so I know the switch is working. I think it is a vacuum actuated system. Any suggestion or helpful hints on how find hte leak or diagnose the problem?


Never seen that setup before.

Either the hubs were automatic…or they were manual. Never seen the hubs that have both.

Last truck I owned that had this type of setup was my GMC S-15 (actually made by Chryco New Process Gear). There was a vacuum activated diaphragm. If there was a vacuum this would prevent the hubs from locking. The diaphragm sat in the fenderwell.

Complete info in your initial post is imperetive.
Year ??

There are many places for vacuum to leak.
Vacuum solenoid.
vacuum tubes/lines, which have a check valve.
Hub seals and o-rings.

On a popularity scale we sell more hub seals & orings than the others.

sorry – 2000 ford 350 90K v-10 - i have replaced the front right Hub.

By HUB , do you mean…
The hub and bearing
the 4x4 locking hub ?

Vacuum needs to be trapped within the hub and bearing area.
The big grease seal AND the outer yellow o-ring are responsible for that in this area.
There is a vacuum reservoir on the right fender with a check valve, a vacuum solenoid on the firewall, and lines between that all need checked for leaks.

any trick for checking these lines? I am asking in the most basic of descriptions, how does one go about checking for leaks?

crap and this the 4x4 locking hub…

I use a hand held vaccume pump that has a gauge on it. You can attach it to each hub and apply vaccume. It should hold, if it drops you have a leak at the hub. If it checks out connect the pump to the main line in the engine compartment and test the entire system.
Hope it helps.