While you are at it it wouldn't be a bad idea to change the filter as well, though if you decide to do that you will need a pan gasket and the filter, though you could do that after the first drain. The check balls are encased in the valve block so as long as you don't take the valve block out you will be fine. Running the car at idle for a while just flushes the fresh fluid and seafoam through the system and cleans the gunk out of the valve block and anything still suspended in the clutch packs, and then that all gets flushed out with the second drain.

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2001 Chevrolet Monte Carlo SuperSport