Problem: One or both rear axle seals leaking on vehicles equipped with vented axles
Cause: Rear axle vent plugged not allowing expanding gases to vent. The expanding gases cause the rear seals to leak.
Solution: Always check the condition of the vent when diagnosing rear axle seal leaks. Check the vent from the outlet fitting at the housing to the one way check valve. Service as necessary.
More Info: While wear and bearing problems often times cause the rear axle seals to start leaking there is another little known cause. Many solid rear axles are vented. They usually use a rubber tube that goes from the vent in the axle housing and is attached to the vehicle’s frame (See Figure 66.1). At the end of the rubber tube is a one way check valve (See Figure 66.2). These devices are designed to allow the expanding gases out, while at the same time not allowing contaminates in.
Figure 66.1 
Figure 66.2