Today: What's Causing Spongy Brakes?

Dear Car Talk

Dear Car Talk | Nov 20, 2014

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.


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