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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 number: AP2617344

There can be a bunch of reason why there is water left in the unit.

I am assuming that the unit with the current plumbing hook up used to drain OK.
Note that there is always some water in the sump as these pumps will burn out their seal if run when dry.

Check that the wash/drain solenoid on the pump left side is pulled down all the way during a drain cycle. Down is drain, up is wash.

Check that the drain hose is clear all the way to the house plumbing.
A common place for clogs is the fitting that goes into the end of the drain hose.

Check that the sump is clean and nothing is wrapped around the chopper.
Be careful as there is often broken glass in the sump.