If you have major leaks in the SLS system the pump may not be able to lift the rear. Make sure the reservoir is full as well. I wouldn't blame the pump just yet. Get the leaks sealed up, and see what happens. Also make look at the arm on the leveling valve. It should be parallel to the ground. If it's not then the system isn't pressurizing. If it is an the rear is sitting low then it may need to be adjusted.

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