Seeking education about Transmission Vacuum Modules

[71CHEVYSHORTBED402]

I came across an NOS modulator. it’s older than our trucks. I like it and may install it, but some questions.

1. The NOS modulator is part number 1361577. I can’t find anything other than the same one was sold long ago, said to be for Olds 442 etc. That said, JEGS refers to 1361577 for their replacement modulator, and they reference 3027223 for the same part, which was the modulator in a 69-72 Blazer 350TH without a 350 engine. I can’t imagine what other block that would be less a 305?, but OK. All other trucks with TH transmissions took a 3027222, so I don’t know, any guesses why the difference 3027222 & 23?

2. I can speak for one difference between 3027222 & 23, per the parts book GM calls the 22 a “Vacuum”, and the 23 an “absolute press” (reference image). That means nothing to me, so there’s my 2nd question. Other than that, written on the box of the NOS reads 1394526, which per other replacement modulators is interchangeable with both 3027222 and 23. Just some background info. The rest of my questions are:

3. The shaft on the NOS that inserts into the transmission has an internal barrel for lack of better term, which slides up and down easily (reference image). It’s much like a PCV valve (i.e., shakes). My old modulator, which I think is original, appears to have the same barrel but it doesn’t budge, even with a magnet. Can anyone elaborate? Personally I think the old one is jammed.

4. I can’t budge the adjustment screw on the NOS. It’s hard to make out, but there may be a very thin seal around the screw. It’s that or just unpainted. Could be rust, but this NOS is in real nice shape. In the image you’ll notice my old modulator has a nut to secure the adjustment position. The NOS modulator does not have one, nor did the other 1361577 I found online. The slot is positioned like my old one, for whatever that means. Either way this doesn’t deter me from using it less the setting may be inadequate for the 400TH I’d install it into. What little I’ve heard is modulators should be good out of the box. What do you think?

5. How exactly do you test these? I tested both valves @ 12 lbs, but I don’t know if that’s adequate.

6. I don’t have a scale, but the NOS might be 1.5 times the weight the old one, it's stout. Feels & sounds like there’s a spring inside, the other doesn’t, though transmission could be preventing it. Perhaps the retainer for a 400TH is inadequate for this new mod., I think it is, but I’m interested to hear what others think.

7. Overall, should I give this NOS a shot in my 400TH? With that I’m going to throw a slider…….

8. The 1971 did not have a return line from the fuel pump to the fuel tank for TH transmissions, but the 1972 did. Contrary to some belief, that return line has to do with TH transmissions, though there could be another component. I know because per the GM parts book the fuel senders and fuel pumps cite TH transmissions for 72, prior to that it didn’t matter. Don’t know why, for all I know it’s because of a variance in TCS or some other govt. program. At any rate, might be something to consider whether this NOS modulator is adequate for this project, especially since the adjustment screw doesn’t seem to want to budge.

There’s always the option to stick with the cheap modulator the transmission shop installed. I like the old stuff better for this ride, they’re better units from start. Input appreciated, thank you……