Looking for some advice before taking the transmission out of the car...

Transmission was rebuilt back in the days in 1998 (by me) with a B&M shiftkit/rebuild kit. In 2014 I changed the VB to a Cheetah auto, otherwise working fine on the street during all these years.
But now after almost 25 years and >40 tmiles it has started to slip in 2-3 upshift when hot and on a really hot day not shifting at all to 3rd. When cold everything works ok, not crisp 2-3 upshift but still hard.
When the oil gets hotter the 2-3 upshift goes weaker until almost gone. Reverse is fine, but when hot more soft shift when engaging.
Have not had any leaks and when checking the fluid it seems ok, not smelling and level also correct.
downshifts are ok, both low speed and kd WOT.

I had a little time the other day so I put 3 hydraulic pressure gauges on the transmission and my -68 Charger up on jack stands, to check line pressure, front and rear servo pressure. But for some unknown reason I could not get any readings from the line pressure and front servo, only rear servo was showing. I tested the two 100PSI gauges with air compressor and then they showed pressure, so I do not understand why they showed 0 while testing the transmission.
So only readings I have for the moment is the rear servo.
In L it was between 60-90+ PSI (idle / throttle pressure max), seems ok, and steady readings. In R it was from 90-240 PSI (idle / 2000rpm), but really shaky on the high mark (220-250psi).
I will try once more with another gauge on the line pressure to see if I can get some readings.

So what do you think ? Some sealing failing when oil hot ? Or has the front clutch been working overtime too long already, so a total rebuild to be done.
Is there anything more that I could investigate while transmission still in car ? It is always more fun to repair when knowing what the actual fault was.

How long do a 727 live on the streets between rebuilds on 3-400 HP cars ? I thought 40t+ miles were a little too early anyhow behind a mild 360 smallblock.