Mine did the exact thing yours was doing. Vacuum assist was fine but stiff pedal in the morning. Been like that for the year and 3 months I owned it. Then one night I was driving to get my girlfriend some ice cream when I came to a light that had like no yellow, hit the brakes, then went to hit them a little more because I was still going a bit fast, and it felt like my foot was on the floor. Lost all power assist, so I really hit it and thankfully they still worked, girlfriend was none too pleased as the belt tightened up around her belly and the baby didn't like that... Thinking "that was really weird" I pull over to check my brake fluid and it's all good, no stuck calipers, nothing is smoking or burning...
Get her ice cream and she wants me to follow her to her friends house because she was paranoid about sliding off the road in the ice. Had to drive back home, then 20 more minutes the opposite way and that's when the "BRAKE" light came on. Every so often, like if you needed to slow down a bit more than coasting it would deplete the assist if you needed to stop, you had to press good and hard...wanted to make sure I could still stop in an emergency so I tried getting the abs to kick on on dry pavement... It's friggin hard to do. I'm 6'3" or so and pushing 270lbs (she got pregnant, I also got pregnant)... so be prepared to stand.
Took it into the dealer who I bought it from (never again) and performed the booster recall (shouldn't you do recalls before selling a vehicle???) and they found that yes the booster is bad and it's because "the TCS/ESP switch is stuck." It wasn't, because I clicked it the day before... But now I let it sit for a full 24 hours and the pedal is still soft like the engine is running, haven't had a stiff pedal yet in the morning. Anyway, fixed under warranty and I declined their esp switch.