Volvo Repair: S80 Volvo No brake power assist, vacuum leaks, volvo s80

Question
Good morning Mark,
I have a 1999 Volvo S80. Twice in the last couple of a days when I start my car for the first time and start backing out of my driveway I have no brake power assist.  I receive no messages.  I turn the car off and restart and the brakes are fine.  I have turn the car off and on after that initial start during the day and so far the problem doesn't return.  It has only happen on the first start of the day.
Any suggestions on what might cause this and how to correct it?
Thank you so much
Don

Answer
  Don, two things, one, check for ANY vacuum leaks and correct. Two, check the vacuum check valve at the brake booster. It may be sludged inside and a proper cleaning will resolve it, or replace it.

  Be aware that in below zero climes, moisture collected in the line leading to the booster can freeze, blocking vacuum to the booster until the car warms enough to melt it, and then the car us fine the rest of the day. Clean that entire line and the check valve. I scratched my head for a long time over this one before I figured it out as I live in Minnesota.

Mark