Hello,
If you jack the car in the air, you can drain the transmission fluid then remove the pan. I would advise buying a new pan gasket if yours is the original cork as it probably won't seal well when re-attached. Also on replacement, be sure to follow the torque spec which I think is 11 ft lbs.
Once the pan is off you can take some thin pieces of sheet metal and stick them above the two pistons. Just cut to size and drill a hole in it to mount on a pan bolt hole.
This is explained on p. 27 of this:
https://packardinfo.com/xoops/html/modules/archive/content.php?ContentID=49 Then take the pistons and valve body off. Disassemble the upper valve body and inspect all the valves and springs. See page 12 of your service manual.
The converter outlet valve was simply deleted in the later first gen Ultramatics, so yours may not even have one. If you do, since your pressure is too high, yours may have some problem. First see if there is any problem in the upper valve body, as the converter inlet valve / relief valve could cause this problem if it sticks; it would supply full pressure even when the direct circuit is engaged. If you reassemble after service and nothing changed, you may want to try the outlet valve modification here:
https://packardinfo.com/xoops/html/downloads/STB/50T-61.pdf You may also want to blow some compressed air through your cooler lines to check for obstructions. Just set the pressure to no higher than 15 PSI so there is no chance to hurt the cooler.
You asked which valve is which. Please see starting on p. 15 in my link above. Also see these charts:
https://packardinfo.com/xoops/html/modules/newbb/dl_attachment.php?attachid=1697223399&post_id=262827 Overall, I think it's most likely that your converter inlet valve is sticking or there is a problem thereabouts.
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