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these are the little, jiggly bastards that screw onto the top your front and rear diffs (or just rear if you are 2WD, don’t get smart with me. I’ll turn this car around!).
They let out pressure when the diffs get hot and expand the gear oil. You need a place for the pressure to go to keep it from blowing past the wheel seals and creating a mess (and loss of some gear oil).
30+ years of road dirt, trail dust, gravel, bits of hay, small woodland creatures and such can build up in/around them causing a problem.
Fits 1979 to 1995 22R / 22RE / 22RTE / 3VZ 2WD and 4WD pick up
fist 1984-1995 1st and 2nd gen 4Runner 22RE / 22RTE and3VZ