I had a brake failure on 2012 Ford Edge several years ago. I researched many places trying to understand why, but never found an answer. Was hoping it is ok to ask about brakes in this forum?
Wondering if it has happened to any 6.0 owners?

Here goes
I was slowing to a stop light when the brake pedal suddenly sunk to the floor. Pumping the pedal did nothing, car did not slow. Luckily I was in the right lane and swerved to the shoulder and coasted to a stop. Immediately opened hood, check brake fluid, it was full, crawled around under to look for leaks. Found none and got a tow back home.

Previously the brake booster? was replaced under recall because a diaphragm tore and caused a very heavy pedal to stop. This was not the same issue.

At home I pulled it apart, found no leaks. Assumed bad master cylinder and replaced it with one from Autonation white bear lake. Replaced all the fluid, bled the wheel cylinders but the pedal would still press right to the floor with little resistance, but the brakes did now slightly slow the vehicle. You couldn’t push it with brakes applied.

Lots more research and found a similar situation that was corrected by driving in reverse and applying the brakes repeatedly getting ABS to engage.
We have a gravel parking lot nearby, so I slowly drove to it. Stopped and tried reverse braking on loose sand. The pedal started getting harder. After half dozen attempts with some ABS activation it became normal.

I have had no further brake problems, but that one took years off my life. What actually was the problem? New master cylinder didn’t fix the issue until I could get ABS to engage.
Thanks

 

It was within the ABS unit, possibly a valve or more likely more than one valve that allowed the brake fluid to pass into and out of the accumulator, taking all the volume you were displacing with the master cylinder that would normally go to the brakes. It would be hard to say it had to be done in reverse gear to free the valves, so they go to a neutral state. You could do that until you're blue in the face on a hard surface. I would speculate that you might have had some braking, enough to get a tire or two to slide on the loose surface to engage the valves electrically. It's possible that using computer-based software, the valves could have been operated to clear the issue too. 2012 would have been past my time to hear about field issues.

The accumulator in an ABS is a chamber that can accept fluid when the ABS blocks the flow from the master but allows the pressurized fluid on the other side of the valve towards the caliper/wheel cylinder to dump into and release the skidding wheel. Then, as the software re-engages the released wheel, a pump repressurized the channel to or slightly above the pressure coming from the master before closing the valve and seeing if the wheel skids or not. This is the pulsation you feel in the pedal, and if done right, you don't drop the pedal as you do with a RABS or RWAL passive system.

A couple of stops from an FMVSS 105 test on a vehicle to show as an example of the system working. These are presented at 13xsec, they operate much faster but it make the graph hard to read.




Normally the front and rear pressures are identical, but on apply the ABS blocks the full pressure as the wheel is starting to skid, dumps into the accumulator pretty hard. It keeps trying to bring the rear pressure up, the saw tooth, but it keeps sensing a wheel skid. The drive is trying to compensate and pulls back, the drop in pedal effort and pedal travel. He come back into it, but the ABS is doing dynamic proportions to the rear pressure, limiting, but still trying to prevent a rear wheel skid. But the ABS keeps trying to get the rear brakes into it, getting the best stopping distance. Finally at the lower speed towards the end of the stop, the driver gives up on the rear brake and pulls back.

But all during this brake application, there is a lot of fluid moving into the accumulator and getting pumped back into the circuit. But if the valve sticks open, the fluid will just go into the accumulator. If the driver backs off, the fluid will get spring loaded back into the master cylinder and into the reservoir, but repumping the brake just goes back past the valve and into the accumulator.


The engineer that worked for me and I though that the ABS suppliers wrote in the programming that if you are dong a series of stops (a FMVSS test), the system would be ready to adjust for the next brake application if there was not a normal brake apply as you would have in the real world.

This stop is a front brake only stop; rear circuit failed. The driver is limiting the pedal effort to about 75lbs of the 150 allowed, then works it higher and the tires get stickier. A front wheel skid starts to occur, The system dumps fluid into the accumulator and pedal effort lowers as the pedal drops. The ABS pump re-pressurizes, pedal travel and pedal effort comes back and the pedal is pushed against the driver's foot, but the next 3 seconds the ABS is just pumping and dumping in and out of the accumulator.

 

I was hoping you would reply and I appreciate it. I will reread the response a few more times to digest it more.
I had assumed the master cylinder failed, until it didn’t fix the soft pedal. Then assumed it was an issue with ABS, but ever since that event (5 years ago) it hasn’t happened. I swung by the local Ford dealer to inquire if there were any recalls or a TSB about this issue. They hadn’t heard of it previously either.

Thanks.

 

I wrote that late at night in the manner I used to write technical papers. But, to quote my former boss (masters from MIT), "no one outside of our industry will understand what you wrote. And sometimes I have a hard time." So you're in good hands. Not that my morning writing will be any different.

Unfortunately, by 2012 and after, I no longer got Ford's warranty data on braking, so I couldn't even tell you if there were instances that didn't rise to a TSB or safety concern. However, you could hunt to see if there were complaints to NHTSA through their public site. There was more than one if you were following instructions.

I found a diagram (there are not many good ones) for the hydraulic control unit of an X pattern braking system, one of the two master cylinder circuits goes to the left front and right rear wheels, the second circuit to the opposite. That's a design unlike our trucks used with some passenger cars to provide better stability, but it depends on the brake bias front to rear.

I've highlighted the potential issue you could have had, something blocking a dump valve (small debris, just like the transmission solenoids) or one of the check valves in the hydraulic pump replenishment output. A reservoir would take the fluid volume that was supposed to actuate the calipers on one circuit.

One of the problems as we got away from the older proportioning valves of years ago is they also incorporated a limiting valve, which moved to block a leaking circuit so the brake pedal would not go low. Today, when you lose a circuit, the pedal travel of only one circuit working has the brake pedal about 1 inch from the floor. In many accidents where one circuit was lost, the drivers say they lost all the brakes. They didn't, but the pedal travel is so unfamiliar that they think they did and do not realize they still had some braking left. You go into shock. It would be a simple thing to set up for driver's ed, dump one circuit so people could get trained for how that feels. Every performance test we did out of my facility required us to test stopping distance with each circuit (not at the same time) and boosted actuation failure. All of us were well versed.

You can see that change in the two graphs I provided last night. Again, the same vehicle, but Stop 5 with its failed circuit, has pedal travel close to 5", where Stop 3 has pedal travel close to 2.75" to 3".

 

It is informative, can’t say I understand all of it, but have the general concept.
This is the video I found that lead me to my final effort before going to a dealer.

Thanks again