87' 300sdl 4th gear issue, or not.

[JohnyKent]

Hey everybody, first post for me here so.
long story short
a bit of intro

My 87 300sdl (DPUK 275hp pump, hx30w Super( euro exhaust manifold) rear 2.47LSD from 560sel)

Experiencing a weird tranmsission issue. (had it since i've bought the car with stock turbo and pump setup too)

so i had this car for about 3 years now, it has well over 250k miles on it if not more, PO had 190k in 2013 and he drove a lot, odo stopped at 198k when i bought it back in 2018.
As of now, i am experiencing same issues i had before. so car drives fine bla bla bla.

4th gear stacks over 3rd and feels really loose, say i do 30mph, part throttle, nothing crazy, about 5-7psi boost (DPUK pump and hx30w super, so really light throttle), at accelerating, when it shifts to 3rd it almost instantly throws it in 4rth, no rpm change as of the torque converter was slipping or just not engaging sufficient, not happening in all other 3 gears since if not hitting 3k rpm in 2nd and 3rd if let off the throttle pedal it keeps the revs up(within 200-300 lower then under load, which i recall is normal behavior for a non locking TC. 4th gear is different, if drop the throttle pedal it would go from say 2.5-2.8k rpm would drop close to idle.

it only acts same as 3rd gear past 3.2-3.5k rpm but that's well past 70mph.

recently( few months ago i did a complete tranny rebuild, new clutches, new gaskets, new seals, new piston seals, everything cleaned out, new oil pump bushing (main reason i had the transmission apart anyway(leaking at front seal))
so i went thru the whole transmission, replaced every maintenance item, as far as i saw, i'm no transmission guy so don't take it as granted lol.
i am a mechanic and i catch wrong stuff when fixing cars.
i did completely take apart k1 and k2 drums to replace the o-rings at the bottom which was already plastic.
same i did with the valve body, everything apart, springs valves everything, cleaned out good (lots of fine gunk coz the filter got sucked in and it pumped that material thru the whole thing).
all the channels and valves and springs looked good.

so i bought a superior shift spring kit also, i replaced one small spring in a big spring train(somewhere in middle of it) supposedly should take care of stacked shifts, i replaced the springs in the k1 and k2 acumulators dampers for a bit stiffer there.
in the rebuild kit which i bought from oregon performance, the rebuild instructions (corteco documentation supposedly) they say all the check ball bearings in the valve body are .215 plastic and one .218 rubber, they all are plastic and .215 . so no clue here, again, i'm not a tranny guy, i just can put together whatever i take apart the same way it was.

so in result my tranny shifts okay, 1>2, 2>3, 3>4 shifts feel much better, under load obviously does it better too, P/N to R/D feels much softer without the harshness that was there before. the 4rth gear loseness is still there, no change in that, and after about 3.2-3.5k rpm the 4rth gear feels fine and engaged "properly" and letting off throttle holds rpms withing 200-300 from the on-load revs. forgot to mention that from the moment i owned the car the vacuum modulator has been replaced(old one wasn't holding vacuum) but the difference with driving no vac and 15hg of vac is almost the same (after rehaul i had it turned all the way clockwise to try to fix the 4rth gear) but the difference is unnoticeable.
and no, i did not check the oil pressure when cranking the vac modulator.
i want to understand if my tranny does what it is supposed to do or not.
Open to any opinions and critics

tranny is a 722.3 321