my ml320 misfires when I touch the brake pedal at idle when stationary,I don't know which cyl it is but the engine hunts really bad until the pedal is released.....the diaphragm in the brake booster appears to be ok.There are two sensors at the front of the booster when I disconnect the plug on the small one it does not misfire but I don't know what the sensor is for...

 

So when you put you foot on the brake it will slow the engine a little .And in doing so will show up your misfire even more . I dont know what the part is your talking about can you put a picture of it please.

 

The engine runs fine at idle.....it only drops cylinders when I touch the brake pedal....I am thinking the sensor I described maybe possibly something to do with the cruise control function that is venting vacuum to atmosphere due to a fault or break in the vacuum hoses that disappear under the manifold.... but I am not sure...I will try and put up a picture if I can...