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Most rear axles have a vent built into them, the original Ford banjos got away without one because of the volume of air between the axle itself and the torque tube. One of the things that is often done with a quick change modification is the addition of a vent on the axle housing. Gears turning and heat can generate a lot of pressure and this pressure will tend to force the gear lube past the seals, thus the need for vents. But again, Ford relied on the volume of air in the torque tube to buffer the pressure in the axle housing.