Soft pedal, grinding or a red brake light? Stop
Find a safe place to pull over, switch off and have help come to the car rather than driving it to a garage. AA New Zealand says a significant brake fluid leak can stop a car braking properly, and that grinding can mean the brakes are worn out. If anyone is hurt or in danger, call 111.
Why does it only shake when you brake?
Pressing the pedal squeezes the pads onto the spinning disc. If the disc has thick and thin patches, that squeeze rises and falls with every turn, and the pulse comes back through the car as a shake. Wikipedia's disc brake entry calls this pedal pulsation when you feel the pedal lift as the thick part passes.
People call these discs 'warped'. The same entry says most discs diagnosed that way are really the result of pad material laid down unevenly on the surface. The fix is much the same either way: the mechanic measures the discs and decides whether they can be machined or need replacing.
Where you feel the shake
| Usually points to | How soon | |
|---|---|---|
| Steering wheel shimmies as you slow | Front discs with uneven thickness | This week |
| Pedal pulses under your foot | A disc rising and falling between the pads | This week |
| Seat or the whole car shudders | Often the rear discs or drums | This week |
| Fast buzzing only in a hard or slippery stop | The ABS doing its job | Normal, unless its light stays on |
| Shakes when cruising as well | A wheel, tyre or suspension fault too | Soon, and slow down meanwhile |
| Shake with grinding or a soft pedal | Worn-out pads or a hydraulic fault | Now: stop driving |
Why did it start after a long hill?
Long descents are hard on brakes. Riding the pedal down a hill road, or a hard stop from motorway speed, heats the discs unevenly. Wikipedia calls the shake that follows hot judder, and says it usually comes from longer, moderate braking from high speed.
Sitting with the pedal pressed hard after a hot stop can also leave a print of pad material on the disc. On a long way down, a lower gear lets the engine do some of the slowing, and the brakes stay cooler.
Is it the ABS?
In an emergency stop or on loose gravel, a rapid buzz or pulse through the pedal is usually the anti-lock brakes working. That's what it's meant to feel like.
If the ABS light stays on, Wikipedia's entry on the system says it usually switches the anti-lock function off until the fault is fixed. The ordinary brakes still work, but you lose that help on a wet or loose surface, so get it looked at soon.
Until the mechanic sees it
Leave more room
A shuddering brake can make stops longer and less smooth. Give yourself extra space to the car in front.
Note when it starts
Only from motorway speed, or at every set of lights? Only after a hill? It's a useful clue.
Feel where it comes through
Steering wheel, pedal or seat. Put that in the note on the card.
Don't leave it for the WoF
Brake operation is on the MTA's list of what a WoF checks, and judder tends to get worse. Sorting it beforehand means one visit, not two.
What should the mechanic measure?
Disc thickness at several points around each disc, and the run-out (how far the disc wobbles as it turns). They should also check the pads, whether the calipers slide freely, and the wheel bearings. A sticking caliper can overheat one disc and start the whole problem again.
Ask whether the discs are still thick enough to machine, against the maker's minimum, or whether new discs make more sense. New pads alone on an uneven disc tend to bring the shake straight back.
What to ask before you say yes
- Which discs are out, front or back, and by how much?
- Machining or new discs, and why?
- Are the calipers sliding freely, or is one sticking?
- Will the pads be replaced to match the discs?
- Is the price in writing, with a call before anything extra?
What you can do, and what's the mechanic's job
You can
- Note when the shake happens and where you feel it.
- Use a lower gear on long downhill stretches.
- Look over the tyres for bulges or uneven wear while the car's parked.
Leave to the mechanic
- All work on the brakes themselves: discs, pads, calipers and fluid.
- Measuring disc thickness and run-out.
- A road test after the job.
What we'd do
We'd send it as a brakes job with a line in the note on where the shake comes through and at what speed. Then we'd ask for the disc measurements and the plan in writing, and we wouldn't agree to new pads alone on a disc that's out.