Driving tests

Signs of bad brakes and when to change them

All brakes have a limited life span due to the extreme conditions under which they operate. They’re essentially small disks of metal responsible for stopping the rotation of wheels connected to 1500kg or more of moving metal. They have to dissipate a lot of kinetic energy and this is done through friction which generates heat. This causes the brake pads and brake disks to wear out, eventually needing replacement.

Most brake issues will be picked up with routine maintenance, but your car or truck will give you plenty of signs that the brakes need attention before they fail catastrophically.

At this point, you’ll need to buy replacement parts. Pads are not expensive but new brake disks are. You’ll replace pads before you replace the disks and it depends on your style of driving as to when they’ll wear out.

We got some advice from The Vehicle Lab as to how to determine whether your brakes need some attention.

1. Squeaking or squealing

When you first put new brake pads on, you might hear the pads working against the brake disks – a kind of rough, grinding sound a bit like sandpaper over wood. This can also happen if your brake disks get rusty. After a short time, the pads are worn smoother or the rust is worn off and this noise reduces. Eventually, though as the brake pads wear down they expose the metallic wear indicators which rub against the disk creating a squealing noise. At this point, you’ve let your brake pads become far too thin to effectively adsorb the heat generated through braking and they should be replaced immediately.

Sometimes you’ll get a small stone caught in the brake calliper which will cause a persistent squealing. This is bad for the disks as it will eventually score a groove in it. A fairly reliable fix for this issue is to find a quiet street or car park, drive fairly quickly in reverse and apply the brakes as hard as you can; this usually ejects the stone out from where it’s become embedded, but take care not to lose control.

Don’t ever apply oil to squeaking brakes.

2. Vibration and wobbling

Vibration through the brake pedal can be caused by one of two things: you’ve applied the brakes so hard that ABS has kicked in (this is more of a buzzing vibration which you will hear), or you have warped or cracked the brake disks through getting them too hot. If it’s the latter, you’ll probably need new disks. Although, minor variances can be fixed by machining the rotors.

On trucks, brake disks which have been exposed to excessive heat will turn blue.

3. Soft brake pedal

When you push the brake pedal it should provide some resistance. If it goes all the way to the floor, you have a problem with your brake fluid. Either it’s very old and has absorbed a lot of water, or you have air in the brake lines due to loss of brake fluid.

You can experience this if you start getting brake fade, in which case you’ll need to stop to let the brakes cool.

4. Burning smell

A burning smell means your brakes are overheating. This either means that the brakes are constantly rubbing (either they are misaligned, or not releasing properly or you are riding along with your left foot on the brake), or you have braked too hard or for too long (e.g. when driving down a long hill).

The way to avoid this is to slow down and use your gears to help with engine braking.

5. Pulling to one side when braking

It’s normal for brake pads to wear evenly on either side. I.e. it’s not normal for the front left brake pad to wear much more quickly than the front right. So, the braking power for each wheel should remain fairly constant.

There are some scenarios where braking performance might be worse on one side. For example, if you are on a surface which is slippery under the left wheels but has a lot of grip under the right wheels, the vehicle could pull to the right if you brake hard. Modern vehicles have a multitude of technology which maximises the braking performance such as anti-lock brakes, electronic brake force distribution and electronic stability control which keeps the vehicle going in a straight line even if the braking ability varies.

In older vehicle, pulling to one side is caused by a brake calliper on one side of the vehicle applying more pressure than on the other sides. This can put you at a risk of spinning as one wheel will lock up earlier than the others. It’s best to get your brakes serviced.

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Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

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