Here is a write-up I did on for Diagnosing the SLS system
Diagnosing your SLS Sytem

Here is the info from the Factory Manual about the SLS system:
http://oregonstate.edu/~lindgrea/DIY/PDFs/32-501.pdf

The most common issues with the SLS is a leaky height valve, or bad accumulators. Sagging over night means your valve needs re-sealing, bouncy or harsh rides mean the accumulators are shot. Not rising when the rear is loaded is usually the pump.

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