Question on Breather verses Pressure Relief Valve and ARB mod

[tundracamper]

I have been trying to understand differential breathers versus the valve on the stock 4Runner. From what I can understand, the 2019 4Runner TRD OR has a pressure relief valve on the top of the differential. From what I've read, this is a one-way relief valve and simply relocating this to the end of a long tube will not allow the differential to "breathe" as it merely relieves high pressure. I am guessing air comes back in through the axle seals (??) as there seems to be no other way in. True "breathing" can be done with the ARB kit, though, which has no valve at the end - just a filter. So, now my questions...

- Will a true breather like the ARB allow the fluid to somewhat evaporate, even if at a slow rate?

- It seems some people claim the front diff breather is a 2-way valve while others claim it's only one-way (i.e. relief). Should I use an ARB on the front diff and TC so they can "breathe" as well? Simply moving a one way valve does no good if rapid cooling in water can't draw in fresh air.

 

[r2m]

Oil cannot evaporate. What the happens is the air in the differential gets very hot and just as we all know for 6th grade science, when heated, air expands and when air cools it constricts causing a vacuum. So all the breather tube is doing is allowing for hot air escape and when the diff cools, allow air back in. If there was not a breather tube, the axle seals would blow out. Longer breather tubes are just if you're submerging your axles all the time or long lengths of time, e.g. going up streams, deep water fording, etc.

When I added my from and rear ARB locking differentials and re-geared my GX I asked about whether or not I need to extend my breathing tubes or add a small bellows (sometimes used in place of an extended tube). I was told that unless I was submerging my axles all the time, don't worry about it.