Some good points here - some not so good. Cold squeal is quite expected - especially in reverse - hot squeal indicates an issue. But not necessarily a bad issue. As mentioned above - brakes designed to be most effective from 150mph to 75 mph (racing and high perf) will be harder than the best choice for 100-10 and will squeal at the lower speeds!
Brakes do NOT work simply by pad rubbing on metal. They BED - that is particles of the pad rub loose and inbed into the pores of the rotor. The bedding in the rotor creates a controlled friction - matching pad to material gives a constant friction over a wide temp range. Without proper bedding that range is very narrow and the friction created is not even through various pedal and speed combos - in other words inconsistent. Creating heat uses up energy - the energy "expense" - the stopping power - is done by that bedding... Rubbing your pad faces on concrete is NOT a good idea - concrete dust is then on\in the pad. That dust then grinds on the rotor and pad accelerating wear - and destroying the eveness of the bedding - if the pad is wearing faster (by adding the concrete dust abrasive) it simply can't make the friction (picture a pad made of adobe - it would crumble away before it made enough heat to slow the vehicle - concrete dust would create a crumble effect at the pad surface). The grind not only removes the bedding from the rotor - it speeds up the pad erosion and thus causes the bedding to run cooler - less brake. Further - cleaning rotors with sandpaper is not a good move. Removing the bedding unevenly (which there is NO WAY to remove it evenly other than turning the rotor) causes different temps to be created rotationally - warpage follows the uneven heating.
Surprised that no one mentioned to check the squealers. There is a tang on the pad that touches the rotor and squeals when pad thickness hits a certain point. Pad thickness is not a "looks fat to me" spec. typically 45% thinner than when new is the replacement point. Pads that "look good" can be gonzoes.
Next check is the pins and sliders, followed by pad backing. Some pads do have a layer on the back - that layer gets worn and you either replace the pads or live with it - the backing goop (never squeal) will erode the layer further - the goop is for UNBACKED pads as mentioned above. Ive seen guys put a new layer on - but for the cost of pads - why?
Being too easy on your brakes can glaze the pads - and they can be rebedded a bit - as above perform several hard applications (varying amount of brakes) from 50-20 (not a complete stop) allowing a cool (approx 5-10 miles at steady speed) between them.
The biggest cause of uncorrectable squeal is the wrong pad. Choosing a "high performance" pad (harder pad material) thinking "more stopping power" is wrong. Pad hardness dictates the speed at which optimum heat is produced. The pad for a race car is designed to make the right amount of heat at >120mph. The heat developed at 75 and below is not enough to stop - and could leave you underbraked (under bedded). On the flip of that running a soft pad too fast will fade the brakes from over bedding.
J Wm Bishop EA, ASADE
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