Vacuum Questions

I have a 93 cutlass supreme and I recently was testing the vacuum, I thought all was well, because the vacuum held steady at idle, moved with accelerating and coasting, I hooked it up through the Cruise controller, I just disconnected that vacuum and hooked in my guage, now I tried the same thing with my brothers car, which is a 90 lumina, same 3.1L engine. and his slowly builds a vacuum, and as you rev the engine, it'll build to about 20 or so, but when i turn the engine off, It holds the vacuum! how is this possible? also Is my car supposed to do that...


the reason i ask is because i have some hesitation issues when cold, and sometimes at operating temp. Im thinking its a vacuum issue. Any info would be great!