That's usually a sign of contamination on the linings. Here's a copy / paste version of a previous reply. Not everything applies to your noise, but look especially at numbers 4, 5, 9, and 10:
Here's a list of common things to watch for that you won't find in the service manuals. They just list the steps in the procedure, not the things that cause problems:
1. Use a large flat blade screwdriver to pry the pistons back into the calipers to make room for the new thicker pads. Doing this will force the brake fluid behind the pistons back up into the master cylinder reservoir and raise the fluid level. Since that room is needed for the fluid is why mechanics will never top off the brake fluid during other routine service such as an oil change. Many do-it-yourselfers remove the calipers then press the pistons back in with a c-clamp. If you find that you MUST use a c-clamp to get the pistons retracted, replace the calipers. There is a ring of dirt or rust built up around the piston causing it to stick on the square-cut seal. That will cause that caliper to not apply during normal to light braking and it will not release after hard braking. We used to rebuild calipers as a part of a conscientious brake job, but today professionally rebuilt calipers cost less than just a new piston.
2. If you DO have to replace the calipers, use a stick between the seat and brake pedal to hold the pedal down about 2 inches. That will prevent the brake fluid from draining out of the master cylinder and will make bleeding much easier.
3. When the calipers are off, don't let them hang by the rubber hoses. Hang them with wire or set them on the steering knuckle where they won't fall down and tear the hoses internally.
4. Use some sandpaper or a flat file on the leading and trailing edges of the pads to remove the sharp edges if they aren't already beveled by the manufacturer. This will eliminate the "fingernails on the blackboard" screeching effect during the break-in period. If squealing can be avoided during the break-in period, the linings usually also won't squeal later.
5. Also to prevent squealing, be absolutely certain to not get any grease on the rotor or pad friction surfaces. That includes fingerprint grease. If you do, simply wash it off with brake parts cleaner before assembling the parts. Once the brake parts become hot from normal stopping, the grease will soak into the linings and into the cast iron rotors, which are porous. It will continue to leach out later causing squealing.
6. Check the back side of the rotors where they contact the hubs for round rust spots that correspond to the access holes in the hub. Those raised spots of rust will hold the rotor away from sitting flush on the hub resulting in a wobble, a pulsing brake pedal, and a shimmy in the steering wheel. Most mechanics know to clean that rust off before they machine the rotors, otherwise they will machine a severe warp into the rotor.
7. Put a light film of high temperature grease on the rotor-to-hub mounting points. This will eliminate a crunching noise when you go around corners. That's a lot more common on older GM front-wheel-drive cars, but it's still good preventative maintenance on any car. I like Chrysler's "Spray White Lube" but other manufacturers have similar products.
8. Check the rubber brake hoses. If the outer casing has cracks, they should be replaced. In the case of a crash, the other guy's lawyers love to find things like that on your car. Even if you weren't at fault, it gives the lawyers something to tell the jury that makes you look partly negligent. This is the kind of stuff professionals have to think about with every car they work on, and it's why we take our cars to them.
9. Apply special high-temperature brake grease to all contact points between the calipers and the pad backing plates, between the calipers and mounts, and on the sliding mounting bolts. One trade name is "Rusty Lube". It is copper-colored. There are other brands that are typically black, but they all contain molybdenum disulfide. There are glues meant to glue the pads to the pistons and calipers to prevent squeals but they never work as intended. The lubricants allow the pads to vibrate freely at a high frequency without transmitting those vibrations to the caliper where they would be amplified.
10. Use the sharp edge of the flat file to scrape rust and impacted dirt from the mounting surfaces that the pads sit on. After they are smooth and clean, coat them with brake grease. If there are grooves worn into those mounts, the pads won't be able to slide sideways freely. That can cause them to hang up and not apply under light braking, and it can cause a clicking noise if the rotor is warped. The pad will click as it slides back and forth over those notches as the rotor turns. Some imports use replaceable thin stainless steel inserts for the pads to ride on. For some other cars those inserts are available as a repair option.
11. Gravity bleeding usually is sufficient when bleeding is necessary. When brake fluid flows from one bleeder screw, close it finger tight, then wait for the next one to flow. If no fluid comes from one of them you might have to "irritate" the brake pedal a little by hand to start the fluid flowing. For the final step, when all of the bleeders are closed, press the brake pedal a few times, but never more than half way to the floor. Open each bleeder once more for a few seconds to expel the few remaining tiny air bubbles. If you must resort to pedal bleeding with a helper, be sure they never press the pedal more than half way to the floor. Corrosion and crud build up in the bottom halves of the bores in the master cylinder where the pistons don't normally travel. Pushing the pedal all the way down runs the lip seals over that crud and can rip them.
12. Don't forget to pump the pedal enough times to push the pistons out of the calipers until the pads contact the rotors before moving the vehicle. Again, never press the pedal more than half way to the floor.
13. If you must add brake fluid, wash your hands first, and use only clean fluid from a new container that was sealed or capped tightly. Brake fluid will suck moisture out of the air. That will lower the boiling point a lot, leading to one type of brake fade, and it will promote corrosion. Do not get any hint of petroleum product in the fluid. That includes engine oil, transmission fluid, power steering fluid, or any kind of axle or other grease. The slightest trace of petroleum product will cause rubber brake parts to swell and become soft and mushy. When the lip seals in the master cylinder grow past the return ports, the trapped fluid will keep the disc brakes applied. At its worst, if you continue driving long enough, plastic wheel covers will melt and the rotors can become red-hot. The only proper repair for contaminated fluid is to replace every part that contains rubber that comes in contact with brake fluid, and to flush and dry all the steel lines. That includes calipers, wheel cylinders, hoses, master cylinder, and the combination valve and / or height-sensing proportioning valve, if used. If you have anti-lock brakes, the hydraulic controller must be replaced too. Contaminated fluid is a very costly but easily avoided repair.
14. Proper break-in is important too. Perform a few fairly hard stops during the test drive to seat the linings but space them far enough apart to prevent overheating them. Even though the rotors are new or machined smooth and the linings are new and flat, microscopically there is going to be a very small area of contact between them. Braking ability will be greatly reduced until the linings wear to match the rotors. Until that happens, usually within the first hundred miles, higher pedal pressure is required to stop the car. That will lead to overheating the linings and very possibly, brake fade. The pedal will still be firm but the car won't slow down. If that happens, simply park the car and let the linings cool down for a couple of hours. Once cooled, the linings will again have their normal friction characteristics.
Friday, September 25th, 2020 AT 10:10 AM
(Merged)
