got it, cutback spring is to soft, would this loss of pressure continue while in 3rd?
I'm going to say "probably"...
The cut back valve is stroked by governor pressure, so it depends on vehicle speed.
Governor pressure rises in proportion to vehicle speed.
The motion of the cutback valve is prevented (resisted) by TV (modulator) pressure...
Governor pressure pushes on the end plate, end of the valve.
Modulator pressure pushes on the spring end.
if modulator pressure is high (full throttle @ low vacuum),
the combination of modulator pressure plus the spring would keep the CB valve against the plate.
Thereby keeping line pressure high.
BUT, if governor pressure were able to overcome the modulator and spring pressure,
the cut back valve would stroke or stay stroked, and decrease line pressure.
Also, if the cut back valve were sticky in its bore, it's possible it could "hang up" in the low pressure position.
Thus I proposed the experiment to block the cut back valve, to make sure it is not responsible for the low pressure in 3rd.
The other thing to look at is the boost valve on the main pressure regulator valve.
It's the aluminum sleeve with the small steel valve inside.
They have been known to wear the inside of the aluminum sleeve, and cause leaks in the boost circuit.