**Solution**
Putting this here for those who don't want to read the whole thread. The problem turned out to be a bad brake booster. The subsequent repair TRW (OES) booster was bad out of the box as well, because that's my luck. Had a small vacuum leak that would cause it to lose vacuum overnight, or consecutive braking actuations.
So TLDR, brake assist loss (and stiff pedal) was caused by a failed booster.
**Original Below**
So there are plenty of threads about this and I'm going to diag the issue in the next couple of days.
But just wanted to post to get some thoughts, and to just vent a little I suppose. I am assuming (you know what they say when you do that) that at 168000 miles the vacuum pump seals have failed on our 2010 X5d and taken out the booster. It doesn't leak externally, but I have not looked at the vaccum line or internals of the booster. I'm still waiting for the tow truck to bring it home from the dealer where it was getting state inspected. I'm guessing it failed during the ABS lockup test being done by the tech as it drove and broke fine on the way there. I have that kind of luck though...
If you've been following along I've been fighting this X5 for the last 2 years. First the trans went out, so I spent a year(+) trying various things, and eventually just replacing the trans. It shifts wonderfully now. Then, SCR inefficiency popped up, reset that, which led to DDE forced no start condition. Spent almost a year trying to figure that out myself, got ISTA running, unknowingly installed counterfeit NOX sensors, then installed real NOX sensors from the dealer. But not before my mileage and reset were used up so I could not continue ISTA test plans. Sent out the DDE, CAS, and key to have the DDE unlocked. And finally got it all back together which brought me to taking it for its state inspection today so I could start driving it again. And list it for sale so it could pay me back some of the love I've given it. Well... I guess it's not ready to leave yet and wants more money, sweat, and tears.
