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Parts for GE GSD4800J01WW / - AppliancePartsPros.com
Unfortunately a parts breakdown is not available for this unit.
But looking through the parts I see that it uses a piston and nut (see below).
Click on the picture for more info on the part.
This is a common cause of water left in the tub. If it does not seal correctly during the drain cycle some of the water goes back into the tub instead of out the drain.
Usually it is at the bottom/back of the tub, behind a screen filter.
It unscrews.
[PART]AP2617344[/PART]