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When you push the brake pedal, brake fluid – a thick liquid that doesn’t get easily compressed under pressure – transfers the force from the pedal through the brake lines and to the brakes. (Aleksej Kostin/Getty Images/iStockphoto)
When you push the brake pedal, brake fluid – a thick liquid that doesn’t get easily compressed under pressure – transfers the force from the pedal through the brake lines and to the brakes. (Aleksej Kostin/Getty Images/iStockphoto)

Driving concern

Gimme a brake! Why air in the line can be deadly Add to ...

I’ve got a 2004 Honda Accord with about 200,000 km on it. The brakes were really rattling so I had a full brake job about a month ago. Since then, the brakes have been really spongy when I’m backing out of the driveway. I took it back to the brake shop and they said yes, there was a bit of air in the line. They bled it and it was still spongy. They say I shouldn’t be worried about it. Is this normal? Or could it be dangerous? – Bill, Edmonton

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“Sorry, them’s the brakes,” isn’t what you want to hear from a mechanic – spongy brakes should always be taken seriously, according to auto experts.

“A spongy brake pedal definitely points to air in the system and yes, it could get progressively worse,” says Mike McGraw, an automotive technology professor at Centennial College in Toronto. “To answer your question, it definitely could be dangerous.”

McGraw says he would bring the car to another shop for a second opinion – and a bleed on the entire system. A bleed forces clean, bubble-free brake fluid through the system and pushes the old fluid and air bubbles out.

Why is air a problem? A quick brake refresher: brakes are hydraulic. When you push the brake pedal, brake fluid – a thick liquid that doesn’t get easily compressed under pressure – transfers the force from the pedal through the brake lines and to the brakes. But if air gets into the brake lines, either from an improper bleed or from a leak, then the force from the pedals compresses the air and not the fluid. That means the energy from your foot isn’t getting to the brakes like it should be.

A Popular Mechanics guide to bleeding brakes likens air in the line to “a very soft spring in the solid column of brake fluid between your foot and the wheels.”

It won’t get better on its own, and it could get worse – eventually, a bunch of small air bubbles in the line will join together to become one big, dangerous bubble.

So your brakes won’t have their normal pressure – and they could fail entirely, McGraw says.

Shops should be taking complaints like yours seriously, says Bert Gregory of Prime CarCare Group, the parent company of Speedy Auto Service.

“It’s not normal, at any time, to have air in the system,” Gregory says. “The shop admitting to there being air in the lines without taking action to bleed the entire system sounds rather strange.”

The original bleeds might not have been done properly, or there could be another problem entirely, McGraw says.

“They could have pushed back the pistons in the front brake system or the rear brake system,” he says. “They could have popped a seal in one of those and it has developed a leak. I’ve seen that happen.”

McGraw says getting a full inspection at another shop is a good idea.

“Not pointing a finger at anybody, but due to the age of the car, something else might have happened,” he says. “Could they have broken a brake line? I don’t know. A full inspection at a second place would not be unwarranted.”

If you have any driving queries for Jason, send him a message at globedrive@globeandmail.com or contact him through Twitter: @JasonTchir

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