Turbo 350 slipping

[1980maliclassic]

Hey guys my 1980 malibu classic has the factory turbo 350 transmission and it just recently started slipping in the shift from 2nd gear to 3rd gear 🙁 . 1st to 2nd is perfect and reverse has no issues at all. I dont know much about transmission diagnostics and was wondering if anyone can provide me with an idea of what the problem may be. I already checked the vaccum line and the modulator is new. Any input would be much appreciated!

 

[Bar50]

There are some pipe plugs on th side of the case to attach a pressure gauge. They easiest thing, is how does the fluid look and smell. If not bright red and no burnt smell....

Another way to word it, there is no external fix to a Turbo-Hydramatic transmission.