I noticed that my 4WD is only working now when I manually lock the hubs in (thankfully they have that options!), and the air blows out the defroster when I shift, so it looks like I have a vacuum leak.

Where are the typical leak points on a 2006 F250? What's the best way to confirm a leak (other than just replacing all of the hose)?

Thanks in advance!

 

Since your tests confirm a vacuum leak (or a weak pump) recommend the following steps.

Obtain a hand vacuum pump (mighty-mite or equivalent). Disconnect the vacuum line at one hub, connect the pump there and draw about 14" of vacuum. Don't go much more than 14". See if the hub holds that vacuum. If the vacuum drops rapidly, there is a leak in that hub. Leak can be from the o-ring at the lockout (easy fix), seal at the ABS sensor (semi-easy fix), or o-ring leak at the inner hub seal (SOB fix).

Check the second hub like the first. If both hubs are OK, the leak is in the line leading to the hubs, or the pump is weak.

Check that the pump can draw a 14" plus vacuum by hooking the pump gage to the line leading to one of the hubs. Turn the key on, then select 4WD on the dashboard selector switch. If the system gives less than 14" of vacuum, repeat this test at the output of the pump. If the pump does not produce 14" or more, the pump is worn out. If the pump gives high vacuum at it's output, but you see low vacuum at the hub tubing connection, then there is a line leak. A strong flashlight should show the damaged line that is causing the leak.

This test is not all that difficult unless you are doing it in freezing weather. Since our 4WD systems only seem to act up in crummy weather, this is probably the case.

Lou Braun

 

Excellent write up! I'll get one of these pumps and try it out. Thanks!!