Just so folks understand how this is supposed to work, I'll give an overview of the system ...

To start with, let's first talk about a traditional brake system. The master cylinder holds the fluid. As the fluid is "replaced" into the pressure side via the return of the rod along it's return stroke, the fluid volume has to be supplied by the reservoir. If that reservoir were completely sealed, it too would have a vacuum which would deter the free and timely flow of fluid into the sealed portion of the system. Hence, there's a tiny hole to vent one side; typically on the cap and goes through a baffled rubber piece.

The 4x4 vacuum system is similar (not same, but close). The vacuum side is on the inboard side of the hub. There is a piston-type movement that the hub uses to "pull" to engage the hub drive when the vacuum is applied. If there were no relief on the other side, then once side of vacuum would actually create vacuum on the opposite side and the movement would stall. In essence the two vacuums would try to balance out and the locking mechanism would not get fully engaged or disengaged. For it to work right, when the vacuum is applied, the other side MUST have the ability to draw atmospheric make-up air volume from somewhere which is NOT restricted. So there's a tiny vent-like design applied to the outboard side.

When the OP washed his truck, it was no big deal. But when he applied and released the 4x4 by activating the vacuum system, it will draw in a tiny bit of outside air as the hub locks, and then expel it again as the system unlocks. The water present with the air will also go in and out. It's unavoidable and expected. But it's also why the systems will eventually get dirt and corrosion inside, and then be slow to engage, or get stuck one way or another, or worse yet, stuck mid-stroke.

There are springs inside the mechamism. Typically these are not needed for the application of 4x4; the vacuum does this. The springs are to encourage the return of the hub lock AFTER the vacuum is ceased (4x4 turned off so the vacuum signal is stopped on inboard side). Then the spring returns the slider to the unlocked position. If done manually, there's a cam that engages with the tuning of the dial that overcomes the springs, and then when the dial is returned to the "unlock" position, the spring resets the system outward again. So either way, the mechanism has to be moved inboard to lock (vacuum or dial cam) and then the spring returns the system to open when vacuum ceases or manual over-ride is dialed "open".

So as to be clear, the vacuum side of the system needs to be fully sealed. However the slave-side of the system has to be open to available pressure to offset that, so the mechanism will activate. If there were vacuum present on the other side, it would not work properly. (Note - vacuum could be used for both sides, but that would complicate the system and both would still need a volume chamber to draw from opposite of the vacuum application).

I recommend taking the system apart once a year, cleaning the guts and applying a VERY light lube (like sewing machine oil or a mist of 3-in-1; not heavy grease or thick oil). If one cannot reasonably turn the locking hub by hand, it's dirty enough to need service.

 

 

I certainly am not an expert, and I could be mistaken, but I thought there was a rubber diaphragm that acted as flexible chamber wall, which lets you apply a vacuum to the master side without creating a vacuum on the opposite side. Also, it should be noted that air is very compressible, and the small movement of the locking mechanism would not create enough vacuum to hinder the function, even if it was sealed completely with a rigid housing. All that is required is enough vacuum to over come it, creating a differential. The factory system is operating around 6"Hg I think, which is really quite a bit. Can either of you elaborate on this?

 

I use the term "piston" loosely here. There is a moving part (several together acting as one whole) that has a seal both on the outer side of the hub, and an inner section as well. Both of these have a rubber o-ring.
several YT vids you can look over.
This is a good one where you can see what I refer to.
Look about 1/2 way through the vid and you'll see the large one they are replacing.


Also this one: the only thing I disagree with is the use of heavy grease; it tends to congeal heavily in very cold weather and not want to shift well. I prefer a lighter lubricant.

 

i wonder if they couldnt of designed a electric locking system. probly more reliable and fully sealed. elock differentials have magnet to pull a side gear back and forth. similar to what they could do in a hub