Just bought this 84 245 recently and am really liking it. It's a 4-speed (w/elec overdrive) and about 180k miles (on a complete rebuild at 140k for some strange reason). The car runs really well, except for this issue with the brakes affecting idle. The problem is only noticeable when the car is cold. It starts up fine, but as soon as I apply the brakes the idle starts to oscillate. The harder I push on the brakes, the wider the oscillation. Sometimes it will just plain kill the engine. When it's warm, there is/may be a slight effect on the idle when applying the brakes, but I'm not 100% certain.
The obvious thing would seem to be a vacuum leak (which might also explain the high NOx numbers I was seeing -- but of course I fixed that by temporarily disabling the Chrysler vacuum ignition advance thingy), but I simply can't find any. Maybe I'm not looking in the right place. The one-way valve in the booster seems to work (I pulled it out and blew through it in one direction but couldn't in the other. Also, after the car has been sitting for a few minutes, there still seems to be a healthy amount of vacuum in the booster (sorry, no quantitative data on that).
Anyway, for some reason I imagined this might be a common problem with these cars and one of you might be able to point me in the right direction.
On a totally unrelated note, never having owned a Volvo before, I'm not sure what they're supposed to sound like. Mine has a bit of a breathy sound to it, like an exhaust leak or a bad manifold gasket. But if I stick my head under the hood I can't seem to localize the source of the sound. I'm wondering if a bit of that is normal, because I'm pretty sure I've heard other Volvos of the same vintage making similar sounds.