Do all brakes screech when worn out?

Question

I have a Nissan Qashqai+2 dci 2010. My mechanic told me that my brakes are going out, especially one of the front ones. He told me that it would be very dangerous to continue driving like this.

The problem is, that I would like to drive to my home country and do the repair there, where it would be a lot cheaper (1000 km away).

Right now I don't hear any noises when braking.

I read that after you start hearing the typical screeching noise, there are another 5000 km in the brakes. Is this true? Can I assume that I have at least another 5000 km in the brakes, if not more?

Could it be, that some brakes don't make the tell-tale sound, when almost completely worn out?

My drive home will be 1000 km of highway, so I won't be doing a lot of braking, but I am not sure, if I should drive it now for another week in the city or avoid driving as much as possible?

Can I tell how much I have left by looking at it, without removing the wheel? Should there be a warning light if it really is about to go out?

Answer 1

Your disk brake pads should have 'wear indicator tabs'. These get closer to the disk rotor surface as the pads wear.

              _{Disk brake pad showing attached wear indicator tab}

A pad with a healthy amount of wear left on it will prevent these tabs from coming in contact with the rotor. When the pad has worn down to the point of replacement, these tabs rub against the rotor during a braking operation. The tabs are typically made from a soft metal that isn't going to produce major scoring on the rotor. This is the first source of minor scratching/squealing/screeching coming from brakes that need replacing.

Of course, you will need someone depressing the brakes while you crawl under the vehicle to inspect any remaining clearance between the wear indicator tabs and the brake rotor. If your brakes are getting close to replacement, this is not something you can 'eyeball' accurately with the brakes not engaged. However, if they have already coming in contact with the rotor, the tabs will show wear on themselves as well.

In that image you should also be able to see recessed rivets attaching the fibrous pad to the backing plate. If you ignore the warning tab, sooner or later you will wear down to the point where the rivets are poking through. These will be the source of major scoring on your brake rotor if you let it get this far.

Brake rotors should be 'turned' by taking a light cut off of them to remove polishing. This can only done until the rotor reaches it functional minimum width. If you have heavy scoring from the rivets making contact with the rotor you might not be able to take enough off the sides without reaching the minimum width. If this is the case, new replacement rotors become part of the repair.

Answer 2

I have only looked at a few service manuals but I have never seen a service manual state that when brake squeal starts, there is ~5km of safe driving. This could or could not be true - but if it is, it is certain to be vehicle specific, pad material specific, driving style specific, etc. The service manuals specify brake material thickness. If the brakes are inspected and measure less than minimum, then they should be changed. A squeal is for you to take note and get them checked.

If you lose all the brake material, you are metal on metal which will give you a longer stopping distance. Even if you drive slowly and carefully, you cannon control the distracted driver pulling out in front of you and you may need the brakes. Hence the mechanics warning. So need to factor in life for this.

If you get to metal on metal, you destroy your rotor. If you destroy the rotor you are replacing 2x rotors and 2x sets of pads which may negate your cost saving of just replacing 2x sets of pads only. Attempting to save a little may cost mode, so something else to consider.

Answer 3

The top answer illustrates a pad with wear indicator very well. A pad like that will make a high-pitched squeak when it is close to being worn out.

If you keep driving, the wear indicator noise may go away if you manager to break the indicator off, bend it, or wear enough of it down. In other words, if you do hear the squeak, the brakes need to be inspected to ascertain their condition.

Not all brake pads have wear indicators. Many 4/6-piston brake systems don't, for example. Do not rely on wear indicator noise to inspect brake pads - inspections should be performed regularly even when you don't think there is anything wrong with the brakes.

A brake pad is comprised of a backing plate and friction material bonded to the backing plate. Normally the pad should be replaced with some of the friction material is still remaining. If you manage to wear through the friction material completely, most pads will produce a screeching noise rather than a squeal. This is pretty bad and generally requires immediate attention. At this point you are also damaging the brake rotors and they generally would need to be replaced as well.

Even when the pad is completely worn out, brakes may still be serviceable, albeit with a much reduced performance. The big danger at this point is pistons physically exiting the caliper - if this happens you will lose all braking ability immediately and this will be catastrophic in most cases.

Can I tell how much I have left by looking at it, without removing the wheel?

No. Brake pads can be worn unevenly, such that the inner edge is completely worn out while the outer edge looks "fine". Conditions like this are generally impossible to detect without physically removing the pads from the vehicle.

My drive home will be 1000 km of highway, so I won't be doing a lot of braking, but I am not sure, if I should drive it now for another week in the city or avoid driving as much as possible?

It is impossible to answer this question without looking at the car. You could wear the brakes more in one week of city driving than in a 1000 km highway trip overnight. A trip in day time could go either way. If your brakes are actually worn out, doing both is quite risky.

My mechanic told me that my brakes are going out, especially one of the front ones. He told me that it would be very dangerous to continue driving like this.

"Going out" is not specific enough to comment on. If you have, for example, a binding caliper, you can wear through the brakes very quickly (since the brakes are effectively always applied). So your mechanic may be right. Or it could be that the situation is not dire and they just want your money. You could either ask for a precise description of the situation (remaining brake pad thickness, caliper binding, rotor wear, etc.) or get a second opinion from another shop and ask them to give you more details.

An anecdote: One time a shop told me my brakes needed to be replaced because they were "rusty". Well, the car just spent the entire winter outdoors and wasn't driven, of course the brakes would be rusty. Those brakes were fine for another couple of years.