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Spongy brake pedal, very hard time tracking down problem

My '07 Ford Focus started having a spongy brake pedal a week or two ago and it’s becoming a bit of a diagnosis nightmare.

Pads, shoes, and rotors are fine. My mechanic said that sometimes a master cylinder may fail internally, so his best guess was to replace it. Fluid level was fine, and there were no apparent leaks otherwise.

He replaced the master cylinder, replaced my fluid, and bled the system. I drove around with it for a day, but it just feels way too spongy… there’s what feels like too much play before braking starts, and I can squish the pedal down really far (a few inches from the floor). It definitely feels like there’s air in the system somewhere…

I brought it back this morning, and his only thought was to bleed it again — I asked about the ABS because that’s seemingly the only other place that the brake lines feed into, but he said that if there was an ABS problem we’d get an error code (no existing or pending errors). Before bringing it back I did a quick stop test to ensure the ABS was working, and it does tremor with a panic stop.

Any other thoughts on what this could be? We’re all basically at wit’s end trying to diagnosis it.

Looks like lots of Focus owners have experienced your problem. I’m also seeing it being common in other Ford products. Some have had good luck replacing the ABS module, but I wouldn’t want to do that until I knew it was faulty, 'cause that’s expensive.

My first thought, assuming (and this is sometimes a tenuous assumption) that your mechanic diagnosed and installed everything correctly and that the parts he installed aren’t defective, would be to inspect the flex lines. If they’ve become weak, they can bulge when you hit the brakes, and then your pedal sinks because you’re having to fill the bulge as well as the caliper.

When you did your stop test and the ABS vibrated, I assume the pedal was still soft? How was the braking itself? Did the car seem to take longer to slow down, or does it stop as well as it did before the problem started?

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Right, the car’s hitting that sweet spot where I’ve done a lot of replacement lately (exhaust, power steering, etc) so I’d rather not jump to that out of desperation when there’s no trouble code or other indication.

Bulging lines is an interesting thought — I’ll have someone pump the brakes and try to see if there’s anything noticeable.

Yeah, the pedal is still soft if I slam on the brakes hard enough to initiate the ABS — the braking itself is rather solid, I can stop quickly, there’s just more pedal movement to get to that point than there used to be.

Now that I think of it there’s another thing: If I pump my brakes with the car off I’ll build pressure and the pedal becomes solid and retains pressure. Pumping the brakes while the car is on doesn’t make any noticeable difference.

Is the brake pedal sinking, when you’re at a stop light?

A thought . . . did the mechanic thoroughly inspect the calipers, hoses, lines and rear wheel cylinders?

Here’s something that I found a few times . . . the caliper was leaking. Yet, it wasn’t possible to see this until the brake pads were removed and the piston was more visible

And the symptoms were identical to a leaking brake master cylinder

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Although if the pedal is solid and retains pressure with the car off, that suggests the absence of a leak.

He replaced the front pads and rear shoes and did check the calipers, said the lines looked fine but I’m sure how thoroughly he checked. Guessing he checked somewhat well because I’m being a pain in the ass about it and brought it back to be bled again.

I got under the car and checked for leaks before even bringing it in and the fluid has been full the entire time, so I don’t think it’s a leak. Going to check tomorrow for bulging lines though (it’s already dark out).

Considering brakes are a life or death matter, you SHOULD be

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I concur, the slow leakdown is the classic symptom of a leaking master cylinder

The problem could even be the abs hydraulic unit . . .

But to condemn it, you had better perform a very thorough diagnosis

That is not always the case

I’m hoping the mechanic bench bled the master cylinder before installation, and hopefully performed the brake bleed procedure using a diaphragm brake bleeder

Another thought . . . did the spongy brake pedal happen before or after the “front pads and rear shoes” as you said?

just a wild guess, and likely not to get it, but once I had the very same symptoms right after pads replacement: pedal would sink half a way before brakes would “catch”, then the second application would be “almost normal”, but 15-20 seconds after it would get spongy again

the root cause was: I messed up hardware clip on one side, it was pushing its spring between the pad and caliper, resulting in pads gap way too bigger than it should be, so it would push the fluid up and would require pumping motion before it would get close, then it would work OK, but in 15-20 seconds it was pushing pad out too much again, so “sponginess” would return

it would be relevant only if OP symptoms coincide with pads replacement, not sure it is the case

One idea, could you be focusing on this so much that its actually performing the same as it always was since new, but it seems worse? See if you can test drive another Focus and compare that one to yours. You might decide it is the same.

If your test drive confirms your Focus really does have a problem, well, the extra movement has to be going somewhere. It’s just a matter of finding out where, eliminating each suspect one by one.

  • air in, or bad caliper (front wheels)
  • air in, or bad wheel cylinder (rear wheels)
  • air in, or bad MC
  • air in, or bad ABS
  • problem with power booster
  • flexible hoses
  • brake pedal hardware problem

This is what I’d do if I had that problem. First I’d test that the booster holds vacuum. Next I’d remove all the wheels, and the brake drums, do a visual inspection. Take a look at the flexible hoses too while the wheels are off. Next re-install the brake drums, leave the wheels off, and verify the rear drums are properly adjusted. Then start the engine & have a helper press on the brake pedal and look at the front calipers for anything unusual happening. Check the flexible hoses for bulging in all four corners too. Nothing found yet? Bleed the brakes again, using a pressure bleeder. Make sure to follow the manufacturers recommendation to the letter, including the procedure to bleed the ABS. Still got nothing? Ok, clamp off 3 of the wheels at their flexible line and test the 4th. Repeat on the next wheel, etc. If one of the wheels is doing it, this test will tell you which one. If there’s no difference, the problem must be ahead of the wheels. Either the ABS, the MC, or the booster. So figure out a way to isolate the MC & booster by preventing brake fluid to flow downstream of the MC, then test. Still nothing? The problem must be the ABS.

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Yeah he bench bled and used a pressure bleeder — and the spongy pedal problem was before replacing pads & shoes, pads/shoes were at about 25% but it was one of those “well I’m here anyway” things.

I did start to think I was too focused on it (am I going crazy?!), but I let someone else drive it and they noticed right away — it’s quite spongy.

I forgot to ask earlier . . .

Do you happen to remember what brand parts your mechanic used?

Eh, if it was the MC, it should be doing it with the car off, too, and it’s not.

Not necessarily with power brakes.

Spongy pedal is usually air in the line. Step lightly on the pedal and it is felt more spongy. Quickly stomp hard on the pedal and the pedal feels less spongy because you compress the air. Some vehicles are easy to bleed the brakes and others will give you a problem. An air bubble in the line can move around when you bleed the brakes and you just might have to jack up one wheel at a time and bleed the brakes that way.

A defective new master cylinder is not out of the question. I worked for hours and days with a new one (maybe it was a rebuilt) for my 1983 Corolla wagon. The store gave me a different one and it worked fine from the get-go. In this case it was not creating any pressure into the brake lines - more serious than spongey - but the underlying reason may have been a similar internal fault.