Moser 12 Bolt fluid issues

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I have a 12 bolt Moser with a 4.10 gear. I am loosing gear oil from the vent tube. It had a 3” tube with the vent cap, when installed. I thought it was to short so I added a 15” tube with the vent cap. I am still loosing fluid. Any idea what is causing this. I have lost about a ½ qt. in approximately 500 miles.

[Hot Black Trans-Am]

You have the vent tube going up and not down?

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It is going up.