This is weird.

2015 3.5 EB
Went to dealer 2 days ago for oil change and for occasionally sticking steering wheel (which they couldn't reproduce and therefore couldn't diagnose). Also got TB and induction cleaning. Ok. Drive home no problem.

Yesterday morning, after rolling start, went WOT and heard a chime. "Check Brake System" on the LCD and the red "Brake" code indicator was on as well. Brakes felt fine but these stayed on for duration of drive. Driving to dealer later in the day, code messages were off. They found nothing out of ordinary and DIDN'T SEE ANYTHING IN THE CODE HISTORY.

Driving back to work from dealer, had to again give it WOT to get up to speed of oncoming traffic. "Ding-ding!!" The codes are back. They were gone for the drive home but returned this morning, again after WOT.

Can someone shed some light on this please?

 

Check your level in the brake fluid reservoir. If low, the sudden G-forces may tip the float just into the zone to register a "low" level.

Could also be an issue with an overly sensitive parking brake switch or some other issue with the ABS system. Let's check the easy stuff first.

 

Dealer *said* they checked fluid, and even if they'd added some and still told me everything was fine, I still got the codes when going WOT after leaving dealership. There is the possibility fluid was needed but not added.

The codes popped on this afternoon when I was idling out of a parking lot, not even on the gas, and yesterday they came on during medium throttle acceleration.

Going back to dealer tomorrow and will leverage the maintenance plan we ponied up $$$ for; won't leave till they find the cause.

 

As expected, it was a sensor. I didn't even know there was a brake booster vacuum sensor, but the braking did seem a wee bit more laborious recently.
$100 deductible and I'll be done tomorrow.

 

UPDATE:
A hose was also found to be in need of repair, so dealer kept truck an additional day (and currently still has).

Great timing for our minivan to start overheating this morning as the wife and I were driving to work (dropping her off first). THAT dealer read a "Manual Shift Overheating" code, which I presume is tranny-related. So with the 98 SHO needing a cam seal (should arrive in mail today), we have 3 cars either currently undriveable or at the dealer. E gads . . .

 

The ecoboost cannot generate enough vacuum in all situations so there is also an electric vacuum pump to supplement the engine vacuum. The sensor is required to measure the vacuum and turn on the electric pump when additional vacuum is required.

 

I guess the brake booster sees a brief but significant drop in available vacuum during onset of WOT until the turbos get spinning. You'd think the vacuum sensor in question would ignore this drop during WOT, but I'm not about to suggest that any more "intelligence" be incorporated into a modern vehicle. Seems that the sensor on my truck was working fine, as it was kicking out the warning and code. I think the real issue was the hose they said needed replacing, which must have been causing more of a vacuum loss than normal.

 

When the turbos start spinning there is no longer any vacuum, it's positive pressure. That's why it has a pump, and I assume check valves.