The ride with failed cells is like massively over-damped, gentle bumps you won't notice it but crash over a speed bump or pothole and it will come down with a crunch.
Pretty sure the cells are steel.
Don't think the levelling will be affected if just the cells are gone?
I believe you can check the cells by poking something blunt into the cell and feeling for the resistance of the rubber diaphragm - not tried it with a good one myself....

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cheers!!