Mechanics

WHAT COULD CAUSE BRAKE CHATTER WHEN COLD AND AT LOW SPEEDS

1999 Chevrolet Suburban • 160,000 miles

I notice it more when vehicle is cold and when I brake while backing out of driveway. After driving awhile it goes away.
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Joemariv@cox.net
March 3, 2012.




Front or rear? Disc or drum brakes in the rear? Have the brakes been inspected recently?

Caradiodoc
Mar 3, 2012.
Sounds like its coming from the front. Rear brakes are drums. Inspected by me only. Rear brake material seems fine. Chatter started after replacing front pads with Ceramics (rotors not turned at this time). After chatter started had rotors removed and turned. Then lapping front ceramic pads with sand paper to remove any contaminates and reassembled. Chatter still present.

Tiny
Joemariv@cox.net
Mar 3, 2012.
The place to start is with the basics. Rather than retype everything, here's a copy / paste version of a previous reply. See if anything here applies, then holler back with comments: ... Here's some other things to prevent problems. To prevent a crunching sound when cornering, put a light coating of high-temperature brake grease between the hub to rotor contact points. This is especially important on older GM fwd cars. Do not get any grease on the pad or rotor friction surfaces. That includes fingerprint grease. Some very picky shops and mechanics will discard pads that get soiled with any kind of grease, but it is usually sufficient to wash all friction surfaces with brake parts cleaner. If this is done before final assembly, there should be no problem. If that grease is there when the parts get hot from normal braking, the grease will soak into the linings and the porous cast iron rotors. It will cause a squeal and never come out.

Another way to prevent squealing brakes is to remove the sharp edge from the leading surfaces of the linings. I used to use a bench grinder, then switched to a flat file. Now I've found it is sufficient to rub the sharp edges on the concrete floor. That removes the "fingernails on the blackboard" screeching. It seems if you can prevent that squeal during the break-in period, they won't squeal later either.

Run a flat file over the pistons and caliper fingers that contact the outer pads. You don't have to shine those surfaces up. The goal is just to be sure there's no dirt or rust that will prevent flat, even contact between the pads' backing plates and the pistons and calipers. Uneven contact will allow the pads to vibrate more than normal. That can set up an audible squeal or chatter. Those same contact points should also be coated with brake grease. That can let the pads vibrate without transferring the noise to the calipers where it will be amplified.

Anyplace the pad backing plates or calipers rest on a metal mounting bracket should also have a light coating of grease. That includes chrome-plated mounting bolts that hold the calipers to the mounts. If those bolts have rust pits or are bent, they should be replaced.

One of the biggest things to watch out for is to not get any petroleum product into the brake fluid. You will likely find the rubber accordion or bladder seals under the reservoir caps have pulled down as the brake fluid left. It is common to pop those seals back into the caps. Years ago, on rear-wheel-drive cars, that was typically done after repacking the wheel bearings with grease. That grease is a petroleum product and is not compatible with brake fluid. Some people ran into trouble after wiping their hands on a shop towel, then using their fingers to pop those seals back into the reservoir caps. That's enough grease contamination to cause rubber parts to swell. When the seals on the pistons in the master cylinder expand, they grow past the return ports and trap brake fluid in the hydraulic system. When the brakes heat up, that trapped fluid expands and applies the brakes even harder. The result is dragging brakes. The only proper repair for contaminated fluid is to replace all parts containing rubber including hoses, calipers, wheel cylinders and the combination valve with its o-rings, and to flush and dry all steel lines. That alone is a very costly and time-consuming repair. If you have anti-lock brakes, the hydraulic controller is full of rubber seals and must be replaced too. To prevent all of this hassle, just be sure to only touch the caps when you've washed your hands, and keep all containers of brake fluid closed to prevent entry of moisture.

It is common for moisture to be absorbed into the hydraulic system over time. That's why most manufacturers recommend replacing the fluid periodically although we rarely do that. You are going to be adding a lot of new fluid to your system. That will reduce the amount of moisture.
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Caradiodoc
Mar 3, 2012.

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