Pressure Released From Fill Plug While Doing Transfer Case Fluid Change

Long-time lurker, occasional poster. Did my first fluid changes on my 2020 OR. Diffs went fine but when I went to remove the fill plug from my transfer case a short rush of air escaped along with maybe 4 oz. of fluid. Not a ton of pressure but pressure nonetheless.

I measured what I got out of the case and it seems like it was maybe overfilled by a few ounces...nothing I don't think would cause any harm. I also checked the breathers by the firewall and the caps move up and down freely.

Is this typical? I can't find much info on it other than breathers could be jammed. The truck had been driven that day but sat for 4 hours or so before I did the fluid changes.

 

Are you sure it was air escaping and not sucking in? I've seen them pull a small vacuum but not pressurize. The breathers should allow air to vent but sometimes not enter back in.

 

I can't wrap my head around why, but these breathers are one way in my experience.
My transfer case and rear diff were both under pressure, rather than vacuum. I replaced the rear diff with a two way when I did the relocate.

 

@airforceb2cc It was definitely air releasing (not a vacuum). I watched a few YouTube DIY of TC changes and a couple of them had some air release (at least it sounded like it but it could very well be vacuum) but mine seemed like a larger volume. I'll be honest and admit I know very little of how these breathers work. I assume they require a certain amount of force to open them (as the fluids expand in the TC).

@jmneill Yeah, how can these build up a vacuum? I thought the breathers let pressure out? Again, I know very little about how these breathers work.

I wonder if the amount of pressure in my TC wasn't enough to push the spring in the breather to release pressure.

 

Temperature fluctuations.
Dip it in a creek crossing - Vacuum
Drive through the Sonoran in summer - Pressure

Again, while I don't get it, I believe the general consensus is these are one way breathers. I could easily suck through mine, but couldn't blow through..
Changed the rear @ 200 miles, so no obstructions could have been present.

 

@jmneill So the breathers let air into the TC (of DIFF)? So any pressure in there has nowhere to go? I thought they were a one way OUT to relieve pressure. Sorry, still trying to get my head around this.

 

That's the way mine were, and many others.
Definitely counterintuitive.
That's what made me swap mine out to 2 ways. The part numbers are out there (on this site) if you do a search.

 

If the system is pressurized (even if just from expanding heated air in the case), that's still working to keep other stuff out.

Seems that's a good thing, as long as the pressure doesn't rise to the level of leaking (or blowing) seals.

 

Not sure about the transfer case, but when I went to do the rear axle on mine, it most certainly had vacuum. I know because as soon as I broke the drain plug loose, I heard a hissing & bubbling noise, all the while no fluid leaked out.

I put a breather hose on it like normally found on most other vehicles. Guess I need to check the others now too (I had read the others (t-case, front diff) were two-way, but this now has me wondering).

 

Yep can confirm both the rear and front on mine were under pressure. Vent was clean on rear. Hell it only had 8k on it when I changed both.

 

Maybe most of us doing our own fluids are performing it on a completely cooled truck so nothing is under pressure. My guess is my TC was just warm enough and below the threshold to trigger the breather to open and release pressure. Ideally I’d assume you don’t want the breather to easily open and allow anything in.

Ran across this video. The breathers she’s showing are two way: https://youtu.be/KroYmPAbOYM?si=aY9hxECDJec6CRO0

 

I replaced the breather on the rear diff once with an arb one and it had preassure in it as well. Hissed out for a few seconds. It had been sitting for hours prior to taking the breather out.

 

Definitely feeling better about my TC after seeing these responses. Thanks everybody!