Soft pedal, grinding or a red brake light? Don't drive on
If the pedal feels spongy or goes lower than usual, the brakes grind, the car pulls hard to one side, or a red brake light comes on, pull over somewhere safe and switch off. Leave the car where it is and have it towed or looked at there. Stopped somewhere exposed? Hazard lights on, everyone out on the side away from the traffic if it's safe, and call roadside assistance if you have it. If anyone is hurt or in danger, call 000. Never try brake work yourself.
Why does the car only shake when you brake?
When you brake, the pads clamp a spinning metal disc behind the wheel. If that disc has thick and thin spots, the pads get pushed in and out with every turn, and that pulse travels up through the car.
People call this warped discs, but RAC's Car Doctor says true warping is rare in modern cars under normal use. It's more likely that friction material from the pads has been deposited unevenly on the disc. Once that's happened, the disc heats unevenly too, and the shudder tends to build.
It doesn't take much. A thickness difference of about 0.17 mm is enough for a driver to feel, so a disc can look fine to the eye and still shake the steering wheel.
Where you feel it is a clue
| What it usually points to | Worth telling the mechanic | |
|---|---|---|
| Steering wheel shakes | The front discs | At what speed it starts |
| Pedal pulses under your foot | Uneven discs, front or rear. A thicker section lifts the pedal slightly as it passes | Whether it's worse after a long downhill |
| Seat or the whole car shudders | Often the rear brakes, or worn suspension parts letting the shake through | Whether it happens at low speed too |
| It also shakes when you're not braking | Probably not the brakes. See car shaking or vibrating | The speed it comes in and fades out |
| Hard pulsing only in an emergency stop | The ABS doing its job, which is normal | Whether it's happening more than it used to |
Is it the ABS?
In a hard stop, or on gravel or a wet road, a rapid pulse through the pedal with a buzzing noise is the anti-lock brakes working. That's normal. If the ABS starts coming on more often and your driving hasn't changed, RAC's advice is to have the car checked, starting with the tyres, because low tread or old, hard rubber has less grip, and the condition of the brake fluid.
Until the mechanic sees it
Leave a bigger gap
Shuddering brakes can take longer to pull you up, so give yourself room.
Brake earlier and more gently
A long, hard brake from highway speed heats the discs most. On long descents, use a lower gear so the brakes aren't doing it all.
Don't sit hard on the pedal after a hot stop
Pads held against a hot disc can leave their shape on it. If you've just come down a long hill, ease your foot off once you've stopped safely, and hold the car with the handbrake or Park if it's level.
Go soon
RACV says not to ignore squealing, grinding or vibrations when braking. RAC adds that driving with faulty brakes is illegal, dangerous and can cause further damage.
What should the mechanic measure?
Ask for numbers, not a guess. A good brake check takes the wheels off and measures disc thickness around the disc and how true it runs, checks the pads, makes sure the caliper (the clamp holding the pads) slides freely, and looks at wheel bearings, tyres and steering joints.
From there the job is either machining the discs or replacing them, usually with new pads. What decides it is how much metal is left: each disc has a minimum thickness set by the maker, and a machined disc has to stay above it. New pads on an uneven disc won't cure a shudder for long.
If you've been told it's warped discs and nothing was measured, RAC's Car Doctor suggests a second opinion in a case like that. Any mechanic who does brake work every day can do this check, and a good one won't mind showing you the readings.
What to ask the mechanic
- What did the discs measure, and what's the maker's minimum thickness?
- Can these discs be machined, or do they need replacing?
- Is the caliper sliding freely on both sides?
- Should the rear brakes be done at the same time?
What you can do, and what's the mechanic's job
You can
- Note the speed it starts and where you feel it.
- Check the tyre pressures when the tyres are cold.
- Tell the mechanic if the wheels were off recently for tyres or a rotation.
- Look at the brake fluid level through the reservoir without opening it.
Leave to the mechanic
- All brake work, including pads, discs, calipers and fluid.
- Measuring the discs and deciding on machining or replacement.
- Tensioning the wheel nuts to the maker's specification.
What we'd do
A light shudder from highway speed with a firm pedal: we'd send the job today, leave more room on the way, and ask for the discs to be measured. A strong shake, or any change in how the pedal feels: we'd stop driving it and ask for a mobile mechanic, or have it towed. Brakes are the one job we'd never put off till next service.