yes there should be some minor fluctuation. The brake booster operates via manifold vaccum. Also a faulty booster could cause a leak and give you issues, however that would be rather unlikely IMO. I work at a dealer and have never had to replace a booster on a cobalt. I would look at the car with you, but I’m up in Plattsburgh. Honestly your best bet really is to PM 88slowmarro and throw him a couple bucks to look at the car for you rather then trying to have the collective minds here on shift diag it through the keyboard. Also CEL on or off?