I am a nonmechanical person. I own a 940 non turbo sedan. During cold starts recently during warmup the car starts fine but then cuts out. It starts right up
but as i get moving the cars rpm swings widely as I apply the brake and then cuts out. When the car warms up it gets better the car does not cut out but the tach fluctuates when appling the brake. I have been told that I need a new brake booster. My question is would a bad brake booster cause the engine to cut out during warmup as well as when I am braking.? What is the purpose of the brake booster. Thanks for your help

The brake booster does just what its name implies: boost the brakes (ie. power brakes). Without it you'd have to stand on the brakes incredibly hard to get breaking.

Could it cause this problem? Yes it could, or it could be just the vacuum valve (plastic thingy on the booster). The brake booster works on vacuum, vacuum which it gets from the intake manifold. If it leaks then what you essentially have is a vacuum leak throwing the mixture off balance and making the car cut out (cold, when mixture is more critical) or run poorly (warm, when it's a bit more tolerant).

I could offer you a deal on a used one, but with shipping (I'm in the Netherlands) that'd probably work out to more expensive than what you can get locally.

Bram

(And yes, I know full well that vacuum isn't a "thing" but rather a "lack-of-thing". For the purpose of explanation it still works to view it in much the same way you'd view pressurized air or fluid: as something that flows from A to B).