Brake noise at the start of each day

Question

There are many question re. brake noise but I don't see one with quite this symptom.

I have hydraulic disk brakes.

• The front brake squeals loudly (and doesn't grip well) at the start of my ride
• This is more-or-less corrected (it works properly), after going downhill about 50..100 metres elevation, after the disk becomes much too hot to touch
• It then remains corrected for the rest of the ride (semi-urban, so using the brake but not continuously)
• The problem recurs after storing the bike for hours (i.e. all day or overnight) -- and it makes no difference where it's stored, indoors or out, day or night
• It doesn't recur after storing it for a shorter time, but long enough for the brakes to cool
• The mechanic at the Local Bike Store tried to clean the pads (saying they're relatively new) and the disks, to no avail

Could it be contamination? Being burned off or scraped off by use? And returning in the course of hours?

If so where might the contamination be coming from?

The mechanic assured me that it couldn't be e.g. the piston leaking (e.g. because it can't leak slow slowly).

Or if it's not from the outside might it be seeping out from inside the pad itself?

Should I suspect the piston seals or something?

The bike is some 12 years / 50000 km old, and the brakes are one of the few components that hasn't yet been replaced (though the brake pads have been frequently). I asked the mechanic whether the brakes might need replacing and he said no, that's not it.

He did replace the hydraulic fluid (because I'd lost pressure, possibly he said from hanging the bike the vertically which I no longer do).

Answer 1

Symptoms - and this statement - "because I'd lost pressure..." point to contaminated brake pads. As identified by @Rеnаud in the comments, pads a porous and absorb fluids. The first bit of braking heats up the pad and burns/wears off the layer of contamination. Over a few hours, oil seeps out to the surface and it starts over.

If this is the case, replacing the brake pads is the only certain method to fix it. Toughly clean the rotor before installing new pads.

If you do not want to replace the pads, you can try cleaning them - soak in something like alcohol or acetone and wipe clean or burn the pads to remove the oil. I have not had much luck with these methods, and the risk of pads failing after such abuse is not worth it to me, so I replace them.

Apart from this, other typical "squealing brake pad" problems such as misaligned caliper etc could be a cause. Caliper realignment is worth a try.

Look at rotors for wear, your mechanic seems to think the brakes are OK, but I would check for myself.

At 50k km, the brakes have done a good job, retiring them, while maybe not needed, might be the easiest approach for a certain fix. New brakes (especially the likes of the Shimano ones that come pre-bleed) are super easy to install. The tradeoff is the value of your time fixing it vs cost of new brakes.

Answer 2

FYI the LBS replaced the rotor (again, not the pad which they said still isn't worn) -- and that seems to have fixed it.

The wear on the old rotor is visible but too small to quantify with only a tape measure, perhaps 0.1 mm.

Maybe that groove worn into the rotor prevented the face of the pad from seating properly, and/or its edge interfered with the edge of the pad.

That doesn't explain why the problem was intermittent -- e.g. I don't see how it might be thermal expansion of the rotor changing the fit, because it isn't hot at all when it hasn't been used for several minutes.

Answer 3

Where I live, I have the same problem with absolute regularity in the months October and November, and to lesser extent, May and April. It affects the 2 bicycles, as well as the 3 different cars.

Whatever vehicle I use in the morning, the brakes squeal down the hill and then settle and I brake quietly afterwards. In some extreme cases when I go back home way too late, brakes sometimes squeal once or twice at the first intersections.

My theory is that the main trigger is the humidity.

The cars and the bicycles are stored in open air, bicycles are protected from the rain but they get their fair share of morning dew or frost in the aforementioned periods.

What is interesting, changing the brake pads in some of the vehicles solves the problem for up to a week. Then the squeal returns. Looks like the water gets somehow in depth of the pad material and makes it noisy. It then evaporates at first heating. New pads are immune to this, maybe they have some hydrophobic coating that they lose in the first ~100km.

The intermittent brake noise is not a big deal for me, but for the sake of the experiment I could try keeping a bicycle in some hotter place for a few days.