Last summer I replaced my rear diff cover with a riddler cover. I filled it up to under the fill plug. But it seems that it has been leaking out of the breather. It has been constantly leaking since summer and I'm tempted to add more fluid. Could I have over filled it that much? Or is something else wrong?

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I won't be able to help much, but do you have the stock breathers, or did you extend them? To me, if it was overfilled, it wouldn't still be leaking for that long.

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Yes you way overfilled the rear diff.They place the fill plug on the Riddler cover(and most other aftermarket covers) for big lifts that tilt the rear diff at some extreme angles.

When warmed up the excessive fluid will blow out the breather but not all at once so it can seem to leak for a very long time.Drain the diff and add the correct amount by volume not till it flows out the fill plug.