Re: XF Brakes Feel Weak..Dead Booster?
Pedal should feel pretty hard before you start the car. Once started, vacuum goes to the booster, and pedal should sink a bit. If there is no change, either booster is cactus or vacuum is not getting to the booster.
If the booster seems OK, could be master cylinder. Has this only just happened, or has it been for a long time. Also, does the car pull to one side or the other? More info may be able to help!