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I have a 2002 GMC Yukon 1500XL. The right rear brake caliper went bad because of a torn rubber boot, which caused the brake pads on that wheel to wear out really quickly. I replaced the caliper, rotor and pads on that wheel. The left rear wheel is still in good shape, with a good caliper, a good rotor with plenty of thickness and 9/32" of brake pad lining on both inner and outer brake pads.

I don't understand why I would need to replace the pads (and rotor) on left rear wheel just because I replaced the pads on the right rear?

I want to leave the left rear as is just in case that caliper were to go bad 10k miles from now.

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There are two main reasons:

  1. So that both sides of the brake equation are equal. This means they are both starting at the same place again.

  2. When you purchase brake pads, they come in sets for both sides. Since you have to purchase new brake pads anyway, you should put them on there or it's a waste of money.

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it's not a waste of money to not put the left rear pads on now. if i put the left rear pads on now and in 10k miles my caliper goes bad and ruins those pads, i have to spend another $47 on rear pads all over again. If i keep the existing left rear pads on and save the new pads until i really need them, at least i have a better chance of not wasting another $47 – Karl Feb 2 at 0:49
    
You may look at it as wasting the $47, but to me, what is the safety of my family, myself, and those around me worth? I'd rather put the fresh pads on there and have piece of mind. Unequal braking is a real phenomenon, whether on the front or back brakes. AND, if you're anything like me, you'll have bought the pads, kept the second set back, then forget where you've put them and have to buy another set anyway. ;-) – Pᴀᴜʟsᴛᴇʀ2 Feb 2 at 0:57
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well, you've got a good point about forgetting where i put them :). sometimes i've got to refind the garage – Karl Feb 2 at 14:45
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@Karl it also means you have to check the rear brakes more often. It would be on my mind that one side may be getting worn down an in need replacing. – HandyHowie Feb 3 at 8:16

The only reason for replacing both at the same time is so that they brake evenly. This is more essential on the front of the vehicle.

If you were to stamp on the brakes in an emergency, you wouldn't want the car to swerve into oncoming cars because the brake on one side of your car worked better than the other.

Edit - make that 3 reasons if you add paulster's.

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The way I understand it, the brake pads will work evenly whether the brake pad linings are equal or not because the caliper bore will have extended out the correct distance so that the pads are always floating right against the rotors. On my right rear, where i just put on new everything, the caliper bore is mostly recessed because the pad linings are new. On my left rear, which still has the old pads with 9/32" on them, the caliper bore is extended further out. In both cases, the pads are floating right against the rotor. I don't see why that would create uneven braking – Karl Feb 2 at 0:56
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@karl Obviously I can't see the two sides to compare them, I am just pointing out something to think about. If you are satisfied that the braking will be equal, then you can make that decision. – HandyHowie Feb 2 at 5:42

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