Happened to me and this is the answer:
Greg's Post: Just got home from a 3 week trip out west. 6200 miles with no major problems with my Promaster. I had 2 issues which cleared themselves and am curious if others have had this happen . issue 1 was a brake warning light flashing on the console for a few minutes . Checked everything ,fluid level ok no leaks at calipers, emergency brake off . Issue 2 was a ETC error and it went into limp mode till we could get to an off ramp. I cut the engine off and restarted and everything was ok, no more issues. Anyone else have any of theses issues and what did you find?
Answer: If you get the generic message "service brakes soon' on a steep grade (15%) its commonly the fluid reservoir level sensor that throws that message even if the reservoir is full. Toggle the key on more even ground and the message clears ---- a bit scary at first to see on steep moutain roads !