For the second day in a row, my Max had absolutely no brake boost for the first few minutes. The pedal was as stiff as a board. It's been 10-deg here for a few mornings. The question: how do I know if it's the booster, master cylinder, or engine vaccum? The brake booster is about $700. Is this a problem that will get worse or will it just go away when it warms up a bit.

 

Are you using your e-brake? Maybe the rear calipers are freezing on you.

 

Thanks for the response. I am not using the e-brake. I should also note that this condition only exists for a few minutes (I assume while the car warms up) and is then fine for the rest of the day - including when I leave work after parking it for about 8 hours.

 

The only guess i could venture is maybe your booster is not holding vacuum and needs a minute or so to build up pressure...