idle fluctuates when brake applied

I have a 22re pu 1994 and it runs good on the highway. Idle fluctuates when I come to a stoplight , and the brake pedal is depressed. When I get off the brake pedal the idle goes back to normal. This increase in idle is not drastic, however it can't be normal for this to be happening. What is going on? Bob:screwy:

vacuum leak?
bad check valve going to the booster?

Your idle is set too high.
Bring it back into spec and the problem will go away.

I have the idle set at 750rpm as indicated on the spec tag under the hood. I have tested and retested all vacuum lines and there is absolutely no leaks. I even used a propane torch not lit of course, and no flucuation in idle. I removed the check valve and replaced it with the same results, and I plugged all ports same same. I am leaning to the booster, and I am guessing that there is a possible leak in the diaphram. I really think that it is a simple fix, but I can't locate the source. Solid mechanic people say that is what the probable source is, however they say it can't be TPS related or the idle would do it constantly. Most I talk to say Tps means throttle only. Brake means vacuum, and if it is , where the heck is the leak! It seems logical that it has to lead to the diaphram in the booster. On the road the truck performs flawlessly, only problem is at a stop w/foot on the brake, take the foot off the brake and all is fine. No other accessories are engaged, and even w/ the ac on and all lights plus, it only does it w/foot on the brake. PERPLEXING TO SAY THE LEAST! Any ideas are of help. The initial start up is fine and it idles back to spec as it should.
I have read all kind of threads about idle problems, and the more I read the more confused I get. I just don't want to throw parts at this thing , because of cost. Well I hope it irons out. Bob
"Remember upon the conduct of each depends the fate of all" Alexander the Great

OK, my answer is because your symptoms are common because of high idle.
The ECM monitors the RPMs, when the idle is higher than normal the ECM thinks you're accelerating, when you hit the brakes the ECM cuts fuel delivery because it knows you hit the brakes, then the idle RPMs come into spec so it restores fuel, the idle goes up, and the cycle repeats. Hence the fluctuations.

I have readjusted the idle and have some results. The idle now fluctuates intermintently. If I turn the ac on it will fluctuate, and if I turn the ac off then turn it back on it idles true. If I put the thing in 4wd unlocked hubs it stops the fluctuation! This is craZY! Does anyone have a idea or answer for this kind of engine idle behavior? What is next? This is fun to say it isn't would make me say the @#$ ##. Bob :banghead:

Turning the AC on will make the idle increase, there's an idle up circuit to overcome the load of the compressor. I'm pretty sure it uses vacum switching.

When you go into/out of 4WD there are vacum motors involved (if you have ADD, which you probably do).

See a trend there ?


You may have a vacum leak somewhere.


just a thought

I had this exact same problem with my 92 toyota pickup. After 6 years I finally figured it out. There are a couple of adjustments on top of the intake manifold area that you can play with and that resolved my problem. You will need to have someone or something push on the brake with the AC on and you can adjust these until it goes away.

Did you figure this one out yet? I've replaced my pneumatic brake booser several times on my '87 pickup. When it has an internal leak, it causes a vacuum leak in the carb (I have a carb). Pumping several times in a row will exacerbate the problem, and verify that it is associated with the power booster.