Today: What's Causing Spongy Brakes?

Tom & Ray

Nov 20, 2014 (Archives)

Dear Tom and Ray:

I am a rural mail carrier, which means I have to use my own vehicle to deliver. I drive a 2005 Scion XA. As you might guess, delivering mail means I am on my brakes all day long. The issue I am having is that by the middle of the day, my brakes will turn very spongy, sometimes as if I have no brakes at all. I can pump the pedal and get enough pressure to stop, but just barely. If I am able to wait a short while, the brakes will firm up a bit, but will go soft as soon as I am back to stopping mailbox to mailbox. The next morning, the brakes will be fine and firm, only to go soft again by midday. It definitely is worse on hotter days. Can you offer any suggestions as to the cause of this and what I can do to keep a firm pedal all day?

-- Chris

RAY: I think your brake pedal needs Cialis, Chris. Isn't that the one that lasts all day?

TOM: It sounds like your brakes are overheating. Those are classic symptoms. The brakes overheat, the brake fluid boils, and you can't stop the car. Then you wait until everything cools down, and you have brakes again.

RAY: So the question is, Has something changed, or did the car always do this?

TOM: If the car has always done this, then your driving probably is a factor. Maybe you're driving with two feet and riding the brake pedal all the time? Or maybe you're lead-footing it from mailbox to mailbox and slamming on the brakes each time?

RAY: This car was not really designed for heavy-duty service like daily mail delivery. That's why they have mail trucks!

TOM: So you may have to adjust your driving, including using a lower gear to slow down so you use the brakes less.

RAY: But if this is a recent phenomenon, then perhaps something is mechanically wrong.

TOM: I would look for the kind of things that could make the brakes act as if your foot is always on the pedal, even when it's not. For instance, a sticky caliper slide or caliper piston could cause the brakes to be partially applied all the time. And then you're compounding that heat by doing stop-and-go driving for hours on end.

RAY: Or the power-brake booster or master cylinder could be faulty, creating constant hydraulic pressure to the brakes.

TOM: My guess is that your problem is caused by some combination of the extreme stop-and-go driving you do and some mechanical problem that has yet to be diagnosed.

RAY: Rifle through your mailbag and see if you can find a flyer for a good brake shop, Chris. Or if you prefer to avoid getting fired, go to www.mechanisfiles.com and search for a shop recommended personally by our readers and listeners. And get it fixed before you start taking out mailboxes.

LeRoi2 • 4 days ago
Chris, the easiest and most obvious cause of brake pedal fade is brake fluid contaminated with water. In extreme service, such as mail delivery, the brake fluid should be changed once per year, and every 2 years is most manufacturers recommendation for regular service.
Brake fluid is by its nature water loving, and water than exists as humidity in the atmosphere contaminates it over time. As the % of the water level in the fluid rises, the fluid begins to boil at a lower temperature than it was designed for, causing fading. Under extreme service, the % of water will cause fading at a lower level, thus sooner, than the every other year replacement schedule. This is why you need to change it least yearly, and possibly more often, depending on how bad the brakes are fading.
If a fresh change of the brake fluid does not make a noticeable improvement, then start working through the brothers list.
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- pghyndman LeRoi2 • a day ago
  Yep!
  As an old-time gearhead, when I read the original "Car Talk" article earlier in the newspaper I was amazed this simple solution was not offered!?!
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hyhybt • 4 days ago
I hope this hasn't made print yet; there's a typo in the link in the last paragraph.
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Lee_T • a day ago
I'm surprised Tom and Ray got focused on overheating brakes and skipped an obvious possibility - an internal leak in the brake master cylinder. In my experience, master cylinders fail more by leaking externally or internally than getting frozen. Sticky brake calipers, whether it's the pistons or sliders, shouldn't be heat-sensitive, they'd do it all the time. So I agree with LeRoi.
Here's my decision tree:
1. jack up the front wheels and see if the wheels turn freely, or just stop on a slight incline in neutral and see if the car rolls.
2. if they're sticky, replace the calipers.
3. if the wheels turn freely, change the brake fluid.
4. if that doesn't help, replace the brake master cylinder.
When I was a mechanic, all the books said power-brake boosters could go bad but I never did see one do that.
Let's not forget that the brake system could be mechanically perfect and it's the driving technique that's overheating the brakes. In that case Chris to switch to a brake fluid with a higher boiling point.

TOTALLY GEEKED!

Today: What's Causing Spongy Brakes?

Tom & Ray