Note: Allpar does not take responsibility for the veracity of any information or opinions here, does not claim expertise, and is not responsible for any consequences. Please proceed at your own risk.
by Bill Watson
Squealing could be caused by a number of things.
1) Dust in the drums. As the shoes wear down, the dust produced builds up in the drums. This could cause squealing when braking.
2) Cheap linings. Low-cost linings do not withstand the heat build up well after repeated stops, especially in city driving. When the linings get hot they are known to squeal. This is also accompanied by reduced braking and increased braking effort on the driver's part. That is, you need more distance to come to a stop and the brake pedal does not feel as firm as it should. And the driver does not feel as confident when braking. Basically, the heat produced with cheap linings causes the linings to glaze. Thus the squealing, and the decreasing braking efficiency.
3) Hard linings. This is a situation more common with buses, trucks, taxis and the like. The harder lining does not break down as easily as the regular linings (also the cheaper linings). Thus there is more squawking when coming to a stop. Also, the drums wear more, as they are taking more of the wear than the linings do compared with using regular linings.
4) Other causes include distorted brake shoes, bent backing plate, broken or weak brake shoe return springs or retaining springs, or saturated linings (caused by leaking cylinders).
There could also be a problem with the differential. A differential goes when the gears get worn out. As to how you would know when it needs replacement, it would either be a failure or increasing noise from the differential. Bearings and seals can be replaced, but they, normally, would not result in the whole unit needing replacement. You can usually check differential fluid levels yourself and should do this at least once a year.
The next time you get your oil changed, make sure they check the differential fluid level. Low fluid level is the major cause of differential failure.
Other possibilities are a rear U-joint or pinion bearing going bad. A shop inspection can find these.
Sometimes, disk brake calipers "stick" or clamp down more firmly than they should. This is a common problem with age, but it can affect brand new calipers. When a brake is squealing, check all four pads to make sure none are unusually worn.
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