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Replacement parts for GE GSD5320D02BB | AppliancePartsPros.com

Looking at the picture it appears that there is another check valve on this "Body Valve" that could be restricting the flow. Is there a check valve in this assembly?
It is probably the Piston and Nut , Item 564 in the "Body Parts" section.
It is basically a spring loaded valve.
It just unscrews from the "Body Valve".
Remove the slotted cover at the inside back of the tub to get at it.
It is not that it restricts the drain flow but if it does not seal correctly during the drain cycle it allows some water to flow back into the tub instead of out the drain.

Is there anything else that could cause this problem?
Could be but this is the most common cause.

I'm stumped.

So now the clutching at straws begins.

I am assuming you have already checked the main pumps sump for debris.
If you look down into the main pump you will see a rubber gasket (don't know what else to call it) in the middle of the flapper valve. Check to make sure it is there.

I didn't see a rubber gasket. The inlet and outlet of the drain pump did not have a flapper valve. Can you be more specific as where to look for this rubber gasket?
It is really not a gasket but I do noy know what to call it.
Look in the "Motor pump"section
Click on the picture for Item 310 "GE pump" picture
A new page will open with better views
Take a look at the center picture, clicking on it will enlarge it.
Take a look down the throat
It looks like a little black shaft sticking out the center ♦of the flapper

Is there a check valve in the discharge line of item # 560? From the picture it appears to have something installed in this line.
Cannot say for sure but I do not think so. You have basically replaced everything in the pump out system perhaps this piece is clogged with crud or something else that I am not aware of. I think I would replace it as a last resort. It is not very expensive and by now you have probably checked everything else at least twice.

Please confirm that all water should be removed from the tub.
I have a unit which does not include the separate pump out pump and it leaves about 1/4 inch of water at the bottom of the tub. Cannot say for sure what yours should leave but can tell you that there should be water in the sump because if you run the pump with the sump dry the bearings/seals will burn out.

Thanks for your perseverance.
Not a problem if nothing else I am stubborn.

Is this flapper valve to stop water flowing to the upper shower head (item # 570) when the tub is draining?
Yes and redirect it out the drain. In older models which use the same pump they did not have a drain pump at all. The main pump did all the wash and drain. I have no idea why they added a separate drain pump, must have had problems with some installations.

Perhaps someone who knows will jump in and enlighten me.