Had our Flex for nearly two years once we hit 22k miles began having brake failures car has been in dealer 5 times they have replaced vacuum booster and check valves. When the car has been parked overnight or for a few hours the brake pedal is very hard the car will not stop.
Ford says this is "normal" operating procedure and we have to warm the car up for five minutes before driving which is totally unacceptable.
Dealer is as frustrated as we are Ford sent out an engineer who said he could not replicate the problem.
We are truly disappointed with Ford.
I got my 2010 Flex SEL w/Ecoboost in Jan/10 and by Mar/10 I was having the very same brake issues.
I had a near accident and had it into Ford in March after the first time it happened and they said they couldn't get it to do the same thing.
As time went on I figured out it happened when the Flex sat for several hours so the next time I took it in I told them they had to let it sit overnight or all day and then try it.
So then they said they got it to do it and replaced my brake vacuum booster.
At first it seemed fixed, but then it started doing it again and when
I complained in a Ford Online survey, Ford sent me a letter and the 800 phone number for their Customer Care group.
I called and they sent out an engineer who never talked to me and said it tested out to spec's and Ford didn't do anything to resolve
The problem has not gone away, although when I start it up in the morning or evening after it has sat for awhile I push very hard on the brake pedal and pump it several times pushing hard and it seems like it helps.
I have had other problems with the Flex.
It is currently at the Dealer as the radio comes on after I turn the car off and leave to go into a store or whatever.
I come back and the radio is on and the other day the battery was dead.
I have been through the same rigamarole with my Flex, the problem has never been fixed and we filed a lemon law complaint against Ford, will see how it goes. We love the car, it just doesn't always stop, my wife and son will not drive it anymore, it has been in an out of the dealer for cumulative weeks now, they have done their best to fix all to no avail. Hopefully Ford can figure this out there are several people like you reporting the same issues. You can see on Fordflex.net
I have a 2010 Flex Eco Boost Limited with the same braking problem.
It has on occasion when first starting cold, have very hard pedal pressure with almost no braking.
I have almost at low speed run into a couple of people coming out of parkades.
It feels like no power boost on the braking but to the extreme.
Requires both feet on the pedal to come to a stop.
Just starting to work this out with Ford.
I first thought maybe I was imagining this issue or getting old.
It has happened to my wife today as well.
I am not a mechanic but I am a mechanical engineer quite versed in auto mechanics.
It only happens when the car is cold.
Yesterday it happened when it was plus 17.
It has happened in a heated underground parkade on a warm day.
I live in Canada and I am worried about what might happen in a cold winter deep freeze blast.
These are complication machines now.
I had an problem when new of unexpected ABS pumping on during slow speed left hand turns.
Rather unnerving trying to turn left hitting the gas to have the brakes blip on for a second with oncoming traffic, sporadically.
Took about a year but they were finally able to get an error code on a steering angle sensor, replaced it and it was fixed.
Only after I could get it to the dealership with the error code still showing.
Suspect this problem will be similar although this is more worrisome, the other problem would fix itself in a second, this problem seems to persist until the car warms up.
Brakes are not an option on any car.
Keep me informed if you get this resolved.
Suggest testing brakes right after start up at very low speeds, pull over and wait until brekes are functional.
So far I do not get any error codes when this happens.
If you do get one note it.
If the brakes start working and an error code persists, drive it to a dealership and do not turn off so they can run diagnostics.
If brakes don't work, get it towed.
I have a 2011 ford f150 ecoboost and experienced the same problem at 22k miles. Like poster above me, I was treated like this problem has never happened ever before. Very frustrating, Almost hit my mailbox then neighbors house first time it happened. Posted on f150 forums and appears the guys there narrowed it down to a valve that keeps pressure from spooling turbo's from affecting the vacuum needed in the booster and actual works against the pedal and why it has so much resistance. It's horribly unsafe and like others, my wife refuses to drive it. To make an already long story short, dealer crashed truck but says not from brakes? Can see link in post here... http://www.f150online.com/forums/2009-2013-f-150/485122-brakes-dont-work-when-cold.html
Just tried taking in my flex, I basically got the denial treatment.
They claim there have been no other claims of this problem.
It didn't do it when thet tried it, they drove it extensively which is basically useless, once it warms up it will never do it.
Looks like it will have to be escalated.
It is doing it more and more now.