Unfortunately, it’s not possible to determine pressures with a diagnostic tool because there are no pressure transducers in the transmission. All you can measure is the current driving the solenoids, but this doesn’t tell you whether or not the solenoids are producing the required pressures.

When you had your valve body assembly out, would you have noticed if there was a crack forming in the upper valve block?





This crack in the S-V 1 to KV-F circuit produces similar symptoms to the ones you are describing.



When warm/hot, the transmission effectively drops into neutral when stationary as the leakage causes the A-clutch to disengage. This only affects 1st gear. Reverse and the other forward gears are unaffected. If the engine revs are raised then the increased pump flow overcomes the leakage and the A-clutch engages, giving you 1st gear. The symptoms are worse when the transmission is hot because the fluid’s reduced viscosity increases the leakage through the crack.

Eventually the crack works its way along the casting until a piece of the valve block breaks off.

https://youtu.be/wUsc-sNFvJU

Phil