2006 S80 just had the brakes and rotors done. A few times (not all of the time) when backing out of my driveway I have experienced no brakes, until I pump them a bit. Brakes are fine when driving, problem did not exist before the brakes where done. Has only happened when reversing after the car has sat overnight.
Thoughts?

 

Though I may suggest solutions, but its a brake job so better either you got to go back to the same mechanic who did the job or find a new one.

Don't drive the car until it fixed.

 

could be the vacuum pump/ switch. Some Volvo models have a vacuum pump which builds up brake booster vacuum quickly on a cold start and runs until engine vacuum builds and takes over. Had to fix this on my S40 - it would take a firm foot to brake while backing but all was normal by the time I got to the bottom of my driveway (100ft or so). Do you hear a whirr noise cycle off/on when idling when you start up?