can brake booster still function with marginal vacuum leak?

[dlb]

i'm tracking down an intermittent slightly high idle on the corolla and am currently suspecting the brake booster. the reason i'm unsure is that when i suck on the hose connected directly to the booster, it develops SOME vacuum, which then bleeds down over the course of 10-20 seconds so that when i release the hose there is just the slightest 'pfft.' i have zero experience with brake boosters so i tried the same test on the booster in my wife's civic and it held vacuum just fine for as long as i kept the hose closed, and makes a big 'PFFFT' when i release it. it's like the corolla's booster is ok with a very small amount of vacuum, but too much vacuum opens up a leak, which causes it to bleed back down to just a small amount.

i don't have any braking problems in the corolla, so that also makes me question the results of my super technical test, but the high idle issue does act like a vacuum leak and i've ruled out all the other potential vacuum leak points, and it seems to me that the booster should hold vacuum just fine without any decrease in pressure. so if anyone can weigh in or suggest any other testing i can do, that would be dandy.

[jimcrazy]

did you plug the hose going to the booster and see how it ildes? may have to get a vacuum pump tester and pump it up to see if it leaks off, I borrowed one from a buddy when checking my dizzy find out real quick if its working or not

[dlb]

yeah, i should get a pump tester. i just might. the car did idle lower with the booster disconnected. that was another thing that indicated a leak to me, but i wasn't sure if boosters are air-tight.

*edit* i just remembered that i got inconsistent results on the idle speed with the booster connected and disconnected, but this is partly because the slightly high idle is inconsistent itself. sometimes, usually when coasting in neutral to a stop, it idles at 1400, then as i apply the brake, it drops to 1200, then slowly continues to drop to its proper resting place at 800-900 rpm. other times, it stays at 1400. other times, it idles fine. but usually it's the first one, 1400 --> brake, rpm down to 1200 --> rpm slowly continue to fall to 800-900.

[Petros]

yes, the brake booster will still work as normal with a small vacuum leak. It should hold a vacuum, not leak down. Likely if it the old diaphragm in the booster is all cracked and starting to steep vacuum, it will not last a lot longer anyway. time to get another one.