Andrew,

Before you start replacing components, make sure the levelling valve is adjusted correctly. You might just need to take in some length of the linkage to compensate for the low height. Those hydraulic pumps don't fail often, but if it is for sure dead, figure about $550 for a new one. Have you looked at the struts for any leaks? If you don't see any externally, they may be leaking internally. With any of these hydrauli suspensions, you always open up a can of worms when you begin working on them.

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Regards,
Aaron