Brake booster or other issue? 1994 940t

[soclosenotnear]

A couple days ago, car started dipping very low rpms at idle. It has not stalled/died; always recovers itself. I realized it was happening when I was pressing the brakes coming down from a cruising speed. Might also be presenting when releasing the brakes abruptly to leave from a red light or similar. I don't hear any hissing and brakes feel fine. Is there any other way to test this. The search function and reading other threads says it would not be the little check valve.

I also found/fixed a couple tiny vacuum leaks. Does not seem to have fixed it.

I realized this morning though, literally right before this issue started, I dropped my gas cap and the gasket broke/crumbled apart. Unsure if this could be related.

Does this sound like a brake booster issue or some other vacuum related issue I'm missing?

 

[OVERDRIVE]

I'm working on a '93 240 right now that has this problem with the booster. I can hear hissing from the booster when my foot is on the brakes which is the giveaway. The car also throws a mixture at idle code.

 

[soclosenotnear]

So, check valve is confirmed working. I bypassed the brake booster itself by capping off the hose and the issue is gone. This indicates the booster itself is the problem, right?

I ask because doing all the little tests I find on youtube make it seem like the booster is working properly.

 

[Khrrck]

the check valve is pointed towards the manifold from the booster. so if the pressure in the manifold rises (if you shut the engine off for example) the booster still has vacuum in it for a while.

If air is leaking into a faulty booster, the check valve will happily let it flow out of the booster into the manifold and mess with your mixture.

I bypassed the brake booster itself by capping off the hose and the issue is gone.

certainly seems like it. although if you capped off the hose at the manifold end, you might try capping off the booster end instead just to make sure that the leak isn't in the hose and/or check valve.

you could also test by shutting off the engine (while parked, brakes released) and seeing if the brake booster still has enough vacuum in it after a minute or two to assist the brakes. if the pedal goes rock hard quickly, it's likely that air is leaking into what should be a sealed vacuum chamber.

 

[soclosenotnear]

Found a brake booster in my shed. Replaced it. My dipping idle/rough running is gone. Also took the opportunity with some extra space to replace the heater control valve that I bypassed when it broke a while back. All good things.

But somehow I must've messed something up with the kick down cable because it's now shifting way too late. Not even sure how that's possible i could have done that. This is one of the only automatic Volvos I've owned so I've never messed with a kick down cable before. Looks pretty straight forward in the manual though. A project for tomorrow.