Ford Fusion brakes stay soft after ABS engages - what's causing this?

I have a 2007 Ford Fusion (I4). Recently the brake pedal goes extremely soft after the anti-lock brake system engages. It can still brake, but I have to press the pedal almost to touching the floor. It will stay soft a while (at least 20 min, probably longer), but I found that if I pump them hard a few times (like while at a red light) it then returns to normal.

My mechanic can’t figure out why it doesn’t immediately return to normal after the ABS engages, and he’s not sure what to replace to fix it. I don’t have the cash right now to just go replacing lots of stuff, hoping it might work.

(He even mentioned the possibility of removing the ABS fuse to prevent the ABS from kicking in and triggering the problem. Though wouldn’t that be a step backward, safety-wise? I always heard that ABS is much more effective than pumping brakes manually…)

Does anyone have any ideas on what’s causing this and how to fix it?


Forgot to say, I bought a car computer obd2 auto scanner gadget that talks to my phone via bluetooth, using the Torque android app, so I can (in theory) use that to look for diagnostic info from the computer, though I’m not completely sure what I should be looking for…


Normally, when the pedal plunges to the floor and can be hardened agian by pumping, there’s air in the system. However, this one seems to have a unique characteristic.

My guess is that the master cylinder is failing. ABS works by rapidly pulsating the brakes vis solenoid activated valves in the hydraulic lines (contained in the ABS modulator). I’m guessing that the pulses in the hydraulic fluid caused by the ABS system may be causing marginal seals on the pistons in the master cylinder to let fluid pass. That would allow your pedal to sink, and have it be directly affected by the ABS. It would also allow the pedal to return to normal without the pulsations.

That’s my wild guess. Others here I’m sure will offer other ideas.

Also, I should add that my mechanic replaced the brake fluid after it happened the first time. Said the stuff he took out was a little dirty, but not too bad. Unfortunately it didn’t fix the problem.

@mlbar, that scanner you’re talking about won’t necessarily pull ABS codes.
OBD2 applies to the engine, not the ABS.
You need to make sure your scanner/code reader will do ABS.

I concur with @thesamemountainbike. It sure sounds like a failing master cylinder.

Don’t remove that ABS fuse. What’s the point of having ABS if you disable it?

You said the pedal goes soft after the ABS engages. I’ve driven hundreds of ABS-equipped cars over the years. And the ABS usually only engaged when I was driving like an idiot and then slamming on the brakes. Are you a pretty hard driver? No offense meant.

ffifrst things first i would bleed the system and put in fresh fluid. the only time my car’s abs comes onj is in the rain and i did something stupid

If “your mechanic” can’t solve a brake problem, It may be time to take the car to a Ford dealership. Hopefully as suggested, replacing the brake fluid which may be contaminated, should help. Personally I would opt for a specialist in this area. In our area, we have a brake service specialist that often gets referrals from the dealer. I would start at the dealer service department if the fluid replacement did not work. Unless ALL the fluid is replaced, moisture will migrate to the brake master cylinder which may not help the problem.

Brake Master Cylinder Tests

Brake Master Cylinder - Bypass Condition Test

Check the brake hydraulic system for leaks or insufficient brake fluid. Repair as necessary.
Observe the brake fluid level in the brake master cylinder reservoir while the brake pedal is slowly applied and released. If the brake fluid level drops when the brake pedal is applied and rises when the brake pedal is released, but the net brake fluid level remains unchanged, the brake master cylinder is bypassing. Repair or install a new brake master cylinder.

Thank you all for your comments.

@thesamemountainbike, @db4690, and @knfenimore, how would you diagnose a failing Master Cylinder? (would prefer to know for sure that this is the problem before I buy a new one). Is it the bypass condition test described by @knfenimore? Are there other diagnostic clues I should look for? (Another diagnostician-mechanic told me you would find fluid or moisture in the cab under the dashboard, but I didn’t quite follow what he was saying.)

@bigmarc and @dagosa, we already replaced the brake fluid, but it didn’t seem to help.

@db4690 and @bigmarc, I don’t believe I am a hard driver (but I guess most people believe that about themselves). Have had the car for 3 yrs now and never had the ABS engage till a couple months ago. Then it’s happened a few times since then. Mostly noticed it trying to drive in the snow, but also when simultaneously turning and slowing in the rain on rough/uneven roads. (We have pretty crappy roads in Philly.)

Thanks for your help and advice!

@mlbar your junky roads and the rain might explain the frequent ABS activations.

Anyways . . . brake fluid in the cab is often associated with a leaky clutch master cylinder, because hydraulic clutches also use brake fluid. And the clutch master cylinder is mounted on the firewall.

But you have the brake booster located between the brake master cylinder and the firewall. So, for brake fluid to be in the cab would mean that the master cylinder is leaking so badly that it’s forming a puddle.

I think that other mechanic got his brake master cylinder and clutch master cylinder confused.

The other diagnostician was incorrect. As a matter of fact your symptoms would suggest a failure of the master cylinder that would not allow fluid to leak out of the system. The leak is strictly internal.

Basically, your MC is a closed cylinder with a piston in it, with a rubber seal around the piston. When yo push the brake pedal, it pushes the piston. The piston pushes the brake fluid, and that pushes the force through the ABS modulator valves and to the calipers. From there, the pressure of the fluid closes the caliper, which clamps on the disc.

What I believe is happening in your case is that as your pedal pushes the piston, the fluid is being allowed to leak past the outside seal on the piston. It stays in the cylinder, Just as bibration causes an object to slide on the table, I suspect that the vibration (pulses) in the brake fluid from the ABS system valves is causing the fluid to leak past the seal when the ABS engages.

Unfortunately, I know of no way to test an MC in the vehicle. Anybody?

@thesamemountainbike and @db4690, thank you for your explanations. Based on my descriptions above, would you estimate that there’s enough evidence pointing to the MC as the culprit that it’s worth pulling the trigger to buy a new MC and replacing it? (Would like to try to replace it myself under the guidance of my retired mechanic neighbor, if it won’t be too hard. (money is tight.))

Or are there still other possible culprits that I should first gather more evidence to exclude them as possible causes?


@mlbar I wouldn’t do that unless you know how to bench bleed a master cylinder and have a diaphragm brake bleeder and the proper Ford adapter.
I’m not a fan of the two guys pump and hold method of brake bleeding, because it literally takes forever to get all of the old brake fluid replaced. Not to mention the fact that you have to refill the reservoir from time to time.

I’m just an amateur driveway mechanic, not a pro at all, but who fixes my own cars, have done so for years, and I’ve always replaced my own brake master cylinders when they fail. I’ve never had much of a problem doing that job. (But I’ve never had to work w/ABS brakes, so my experience may not apply to you.) For my non-ABS cars, I’d rate the job as about as complicated as doing a tune-up. If that is of any help. If you’ve never done brake work before, definitely get assistance from somebody who has. Correctly functioning brakes are pretty important.

If you decide to do it yourself (with the help of your neighbor), be sure to follow the instructions that come with the new unit. On most of them, you have to bench bleed the master cylinder before you install it on the car. After you install the MC, you’ll have to bleed the brake hydraulic system.

I have the same problem with an 08. Bled the breaks, went in reverse at 10+mph and slammed on the breaks. Problem went away. Still don’t know what caused it.

Ray, you should be aware that this is a thread from mid January. I’m sure the OP has long since changed out the MC and has forgotten the thread.

But thanks for joining us anyway. Help is always appreciated.

This is a common well documented problem. I bought a salvage car and thought that may be the reason but it is the Hydraulic control unit that is most likely the culprit. I’m not sure if there is any solid fix for the problem but NHTSA is investigating this. You should report it to them so we can get a recall action