OK, I'm up now. The word dashpot might not have been the best choice of words in this case.
On some cars, the engine vacuum level drops when going up hills, accelerating or passing cars (think two-lane passing) and what happens is there is not enough vacuum supply to keep the A/C vacuum doors open.
There is actually two fixes:
One is to insert a vacuum restrictor (orifice) into the vacuum source hose, which is what is being referred here. These come in many shapes and the ones made 30 years ago are maybe three times larger than these examples. The color points to the size of the orifice inside.
The other fix is to install a vacuum canister. This one is from '68-70 B-bodies. It looks like a large repurposed vegetable can (~6" by 6") with a welded-on mounting bracket, and this one has a two vacuum hose fittings. Behind the hose fittings is a one-way check valve. This is the best fix – but is also the most expensive.
I have one (still in my garage) from my '68 Charger – but it is two (or maybe three) times larger sense it had vacuum operated headlight doors.
This is what Chrysler used starting in the early '70's – which I refer to as the vacuum ball (it looks like a ball with a mounting bracket). This version is still being made by Dorman (I think). It is about 6" in diameter (I think).
What is interesting is my '97 Dodge diesel pickup (and possibly some of your diesel pickups) still uses vacuum A/C controls and vacuum doors - in many cases much like your Fifth Ave. Diesels do not make engine vacuum – so to operate the A/C and power brakes, Dodge installed an small mechanical vacuum pump to operate these items. A simple system and much less complex than electric A/C doors and hydroboost for the power brakes.
The mechanical vacuum pumps look somewhat like a mechanical fuel pump (work off of the same principal).
Getting back to basics. Vacuum is simple and mostly trouble free. It does suffer from vacuum leaks (the plastic pipe/hose tends to crack/break/leak. Vacuum hose is better but more expensive. It tends to get damaged from oil (oil and rubber don't mix well).
High performance engines and diesels toss a curve ball into the vacuum supply and need a separate pump to get things done. There are a couple of common inexpensive 12 volt vacuum on used nowadays to replace the gasoline engine source for those guys. One of many examples are:
If considering one, I'd recommend going to a Pull-a-Part and get it. and the pigtail, for (maybe) $10.
JW, did this help you any?
BudW