Rear diff breather.....

I would say true, but at so low of a concentration it should be meaningless. It would only be a drop at a time into the diff. A good long run on the road would easily burn that off in the oil. The truck had other breathers with hoses installed too.

 

Correct, theory is as you drive, the differential heats up and the air inside expands. If there is nowhere for the air to go, you’ll build up pressure in the differential and risk blowing out a seal or push oil out which is not good for your drum brakes. The one way valve lets this air escape so this does not happen. Eventually you stop, the differential cools down and air contracts creating a vacuum in the differential. Air gets sucked in past the seals which are more tolerant to suction than pressure. Everything fine because your seals should be high and dry when you park the truck.

The problem arises if you are fording a stream deep enough that front differential seal is submerged. The cold water suddenly cools the axle and internal air contracts creating a suction. If air cannot freely flow into the differential through a high and dry, two way breather, then water will get sucked in through the seals.

Hypothetically, if the air in the differential is at 200 F and gets suddenly cooled to 60 F then the air will try to contract to 80% of original volume which it can do if it sucks in 20% equivalent volume of water, i.e. we’re talking about more than a few drops. Actual numbers will be less given there is a lot of hot metal in the differential not in contact with outer walls so it will take more than a few minutes for the air temperature to drop to water temperature.

 

Easy to install and you have options on where you want to install the breather. I installed it in the bed cubby.

 

I just went thru hurricane Ian here in central Florida. Had to drive thru 1-3 feet of water at times. I bought the ARB breather kit a year ago and never got around to installing it. It is on my list of things to do now. Wish I had it done before.