Hmmm, I'm stumped

Well, here's a challenge for ya engine folks. This problem has been persisting since I bought the truck and other than being annoying when at a redlight, and probably degrading my fuel economy, the trucks worked just fine.

Here's my stumper.

When the engine is running, truck is IN GEAR, and the brake pedal is depressed, the engine tends to idle rough. When you let off the brake pedal, the roughness of idle vanishes.

As best as I can trace it, it appears to be vacuum related, but, here's the stumper, I can't find a vacuum leak. The intake manifold is air tight, and after going through 3 cans of starter fluid, I haven't been able to find it in the engine compartment.

So, where should I look, or is this another problem, like some vacuum controled dodad sticking?

I'm stumped, and not certain where to look.

In case yer wondering

Truck
1992 Dodge W-250, 5.9L TBI engine. 4x4, automatic.

Check your tranny fluid level, if it's low then that could be the cause.

here's one other bit to the puzzle that I forgot to mention. In gear, or not in gear, brakes on or not, the TBI has a tendancy to up the engine's rpm for a couple seconds every few minutes like by 200-300.

TPS or cylinder misfire.

your trannie fluid is low or the trannie its self may be going out because it is happening to my 1988 Dodge Ram Custom even though it's old as me

Actually, it seems to be related to the map sensor. Upon forcing the truck to go to preprogrammed defaults by unhooking it, the rough idle vanished. The difference between being in gear vs in park is that the engine is under load.

I'll be checking this by temporarily hooking up a new map sensor today, I definitely suspect it as the culprit because after unhooking it for a while and running the truck, then reconnecting it, the engine already saw an improvement.

Ok ,let me see if I got this.It idles rough when you step on the brakes.And you think it might be a vacuum leak.And there is probably a big vacuum hose going to the brake booster.You don't happen to hear any loud hissing noise from under the dash when you step on the brakes,do you?

Nope, no hissing, wasn't a vacuum leak either, it was a map sensor sending faulting data and mixing things wrong.