I believe the 520 part is the plugged screen I found in my 722.3 Transmission. I am attaching the picture from my transmission manual where I made notes when I cleaned it out. Only one of the screens in my picture were plugged, but it was very obvious when I removed the rear valve body. In the picture what I call the rear valve body is called the lower cover.

On a separate occasion, I added a Superior Shift Kit, Part Number K722-A-D / S68165D. This kit is for both 722.3 and 722.4 transmissions. The kit along with a new modulator was a great improvement! It requires the dis-assembly of portions of the valve body to replace springs.

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Well, it remains a mystery. I should revise my comments above about Sun Valley.

I've been talking to them about this. The tranny is out of warranty, but they are being most helpful. They are sending another valve body for exchange to rule that out. While my mechanic is changing the valve body, I'll have him check out the screens to which you refer. Sun Valley has also indicated that they will carry on with the help with the intent of getting it fixed, which seems a good recommendation. I should mention that I did not solicit the new valve body.

I'm not optimistic about either the valve body or the screens, though I continue to maintain hope. The tranny continues to work properly for the first few miles after a dead cold start. I think that indicates that it works as long as the fluid is cold or not at operating temperature. Seems like that might indicate that something is expanding with heat and freezing (if you will!) at operating temperature. Come think of it, maybe the fluid thins out at temperature and the restricted flow through one of the screens prohibits proper pressure maintenance. I noticed this morning that it "fluttered" when shifting into fourth when I first went out today.

Wish you the best of luck determining the problem.

I chased mine for months. I had two pressure gauges hooked up to the pressure ports on the case and monitored the line pressure / modulator pressure while I drove.
I also had a tee in the vacuum line to the modulator with a line run through the firewall to the console. I had a bleed valve and vacuum gauge hooked to this line. I could adjust the vacuum, the modulator saw, and monitor it by bleeding off vacuum. As I bleed off vacuum the modulated line pressure would rise. This would overcome the restriction and let the trans shift. Of course I did not know at the time why the trans would go ahead and shift other than I knew I was raising the pressure. I think this is probably when I also realized that manually shifting it also raised the pressure. But it finally got to where it would no longer shift. I was in process of buying a rebuild kit when I found the forum that suggested to look for the restricted screens.

With all of the above being stated, have you considered the vacuum modulator? If it is not working / adjusted properly or if it is not seeing the proper regulated vacuum signal, the trans will not shift properly. Also there is a "vacuum valve box / regulator" hooked to the throttle cable down near the injector pump that regulates the vacuum to the modulator (much like my in car adjustable bleed valve did). If this regulator is not bleeding away vacuum from the modulator, the trans does not know to shift.

And as others have stated, if the bowden cable is not working properly, then you will have shifting issues as well.

Alot to take in, here in this reply. Hope it helps.

Let us know what you learn about this problem.