I'll file this under "stupid" ...
So my '93 C3500 that I recently dropped, began blowing gear oil out of the rear end. I, (and some of y'all) figured the likely suspect is a clogged breather. I had to think about it a little and ya know, I drove this truck home almost 200 miles from buying it and it didn't leak a drop. Didn't seem to leak until after I lowered it. Never noticed the leak until I drove it across town to the muffler shop, tailpipes chopped off, dual Flowmaster outlets aimed right at the passenger side axle tube, on a melting hot Houston summer day.
I've left it alone until tonight because I've got so much other stuff going on. Well tonight I figured, let me at least take a look. Rolled the bed mat over and looked through the hole in the bed for the old gooseneck plate.. and I think I figured it out.
Breather appears to not be clogged. I didn't bother removing it yet. It's soaking wet with gear oil and it has run down the hose several inches in the process. It's all over the bracket, the driver side shock, brake hard line, inside the rear bumper, and sprayed all over the tailgate.
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A closer look gives some insight on 100% what I believe the problem is. The breather is sitting lower than the top of the differential, and the hose itself dips down to the centerline of the axle tube.
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I haven't pulled the fill plug yet to see what the gear oil level is (surely lower than it was since plenty of it sprayed out) but it's not hard for me to imagine the gear oil traveled up the hose because it really didn't have much of a choice. Am I nuts?
I really don't have a lot of room to relocate the breather higher up, but I think moving it to where the hose can't dip down that low, and positioning the breather as high as I can get it, will solve the problem. Truck's got a lot of drop vs. stock height.
Richard