There are subtle differences aver the years but , the basic thing is to bleed off the vacuum signal to the modulator as the throttle is depressed until it reaches
zero as the accellerator touches the floor .
As you say your 1980 shifts fine , it is time to stop screwing with it before you " fix it 'till it's broken "
.
The round yellow thing is a one way check valve that should not be open on one nipple .
Testing vacuum devices , valves and actuators , modulators etc. is dead simple : connect a bit of good hose and place your fingers over the other nipples and suck on the hose with your mouth ~ if it doesn't hold vacuum when you p[lace your tounge over the hose end , it's bad .
Or , buy a Mity-Vac tool , it is simple to use and lets you know how well the vacuum pump is working and can be rigged into the tranny's modulator line with a 'T' for proper road testing and adjusting .