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Was checking the rear brakes yesterday and took a look at the axle vent.
It was stuck and didn't move so I sprayed it with brake cleaner and brushed any crap off with a toothbrush and it freed up and moves up and down when you push it with your finger.
Manual says if it moves up and down freely it's ok.
Has some kind of spring loaded jiggle cap on top.
No idea how long it was stuck but this original 8.8 diff has never leaked a drop so I guess it was venting.
Sound ok and good to go?
Yup, no worries. More often than not, they stick up (vented) since they done really move.
I had no idea they move up and down until I looked in the manual later on after cleaning it.
I thought at the time it might just rotate,(which it also does now).
With the dirt around it I couldn't see any path for air flow past the cap.
Best to remove / unscrew so you can blow through. Some vent via an internal spring loaded rubber valve, very common for them to jam shut n cause a problem. If it's that style, easy to pry the cap off and remove the guts.
Can it still jam shut with the vent cap in the all the way up position?
If it's the style with an internal valve, yes.
Looks like the one in the link.
https://www.cjponyparts.com/ford-rea...2014/p/HW3281/
They all look the same from the outside, cap needs to be removed / pried off to see the valve (replacements come valve only). Valve was added as part of mandated emissions to prevent differential / transmissions from venting into the air.
I will leave it as it is at this point as it looks to be working fine.
Drove it today and nothing leaking from the vent
If there was a problem with the vent there probably would have been a axle seal leak by now after 32 years.