Vacuum modulator

My 77 Cadillac doesn’t upshift to 2nd gear until I’m doing 50mph/80kph. When it finally upshifts, it bangs into gear as if something pushes the car from behind. It also idles at a very high RPM, which scarily enough rises when still cold, until I switch it to D. I believe these are both vacuum related issues. I know that the vacuum modulator regulates the shifting points. A problem with the vacuum modulator or the vacuum lines could cause the above issues.

However, when I run my column-mounted shifter through the gears at a stand still (from P to R to N to D, etc), the whole car moves. It really bangs into the gears. They all work, but it seems as if the angle and the height of the car’s body move depending on which gear I’m in. Each gear runs at a different RPM as well.

My question is: does vacuum affect the ‘roughness’ of changing gears to P/R/N/D? So basically when you are at a standstill.

I know that it affects the roughness of upshifting in D (1 to 2, 2 to 3, etc). But does vacuum play a role in changing stationary gears?

EDIT: I realize my title isn’t really accurate. It should be something along the lines of “Effect of vacuum on changing years”. And yes, basically everything in this car is regulated mechanically as opposed to electronically.

submitted by /u/Eelluminati
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