Brakes - booster or master cylinder?

My 09 9-5 2.3t has been showing more signs of age lately and may be coming to the end of the line. I have notices on a few occasions lately that when I start the car and begin to move slowly, the brakes don't seem to grab hard and the car is slow to stop. When I get in, the brake is high off the floor. When i start the car, it moves down a bit. When I start to travel in earnest, the brakes return to normal. The first time I noticed this I stopped the car and checked the fluid level. It was full. When I returned to the car the brakes were fine. I am guessing that the master cylinder is failing, but when I look at the WIS I see there is booster. Can this be failing or would they fail together? The odd thing is that once travelling, all seems normal. Anyone have any thoughts?

Yes master cylinders fail, brake boosters will fail and really should flush the brake fluid at least once a decade or so. Fluid flush is the cheapest possible solution to the sound of this problem.

That's definitely worth a try. I've only owned it for 4 years and replaced the rear calipers when I got it. I haven't touched the fronts.