That's usually due to improper prep of the new pads. Professionals do a lot of things to prevent noises, and there's a lot of things we can accidentally do to cause them.
First of all, if you got any grease or assembly lube on the friction surfaces, it must be washed off with brake parts cleaner before those parts get hot. Once they get hot, the oils will soak into the porous linings and cast iron rotors, and never come out. Even fingerprint grease must be washed off.
The leading edges of the linings should be beveled to prevent the fingernails-on-the-blackboard squealing. I take a few passes with a flat file.
The flat file must also be used on the piston and on the caliper fingers where those parts contact the pad backing plates. You don't have to shine them up. You're just insuring there's no high points from dirt or rust. The pads have to sit squarely against those points, otherwise the normal vibration increases to the point you can hear it. Those contact points must also be lubricated with high-temperature brake grease such as "Rusty Lube" or a similar product. The caliper slides and mounting bolts must be coated with that too but don't get any on the rotors or linings.
If you didn't have the rotors machined, there's no telling what can happen. Replacement linings are rarely bonded to the backing plates in exactly the same position, so new ones can ride on ridges of rust left from the old pads. You must also scrub the center of the rotors and the mating surfaces on the hubs, and coat them with brake grease.
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Thursday, January 31st, 2013 AT 11:04 PM