rebuilding an air-over-hydraulic unit?

I got an 8-ton long-throw air-over-hydraulic jack, an engine hoist one. It's been good to me but now it no longer works with air. The air motor turns over,
but seems stiff, but nothing else happens at all. Sometimes the air motor doesn't even turn over, it just sits there hissing.
Anyone know how to rebuild these things?
Grant
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Grant, have you checked http://www.harborfreight.com/manuals/94000-94999/94562.pdf
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