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DJDD

3 years of brake wear in 8 months???

Question

2009 Silverado 1500 Z71 4x4 Crew Cab, 68,259 miles when replaced. Current miles are 84,409.

 

July of last year I replaced the brakes all the way around, had the front rotors turned and bought new drums for the rear. Fast forward to this month and I just replaced the front rotors and brakes because both sides had worn a groove into the rotor and there was no pad left on the outside brake pad. Good pressure on the calipers, almost too much pressure, and the slide bolts nor the caliper piston appears to be stuck. When sitting with the engine on and foot off of the brake the pads are touching the rotor. Something isn't right but I can't figure out what. Will probably be taking it to the dealer on Friday.

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Brakes only lasting a third of what the OEM brakes lasted is not unusual. OEM brakes were installed with every component being brand new, right down to fluid, steel lines, flex hoses, metering valve and proportioning valve(last two make up combination valve). Also, every flex point was properly greased, and again, brand new. Had you replace all parts with brand new, you could expect to get same results the first brakes gave.

 

Generally, if the brake pad wear is uneven when using calipers that only have piston(s) on one side, it means the sliders are not completely free. During braking, the piston will be pushed outwards, pushing the brake pad into the rotor, then it becomes easier to make the caliper slide away from the piston, and in doing so, it will pull the outside pad into the rotor.

 

Some background info on how disc brakes are applied and released, and why the strange wear. Some may find this unnecessary, I think it makes things a lot easier to understand if you know what is actually happening.

 

Disc brake pads will always make contact with the rotor. Keep in mind that the surface of the brake pad is not perfectly flat/smooth or as hard as the face of the rotor. Similar to piece of un-polished, finished wood, and a glass surface. There is not really anything in the caliper to move the caliper once the brake is released, other than the distortion of the rubber seals on the guides(if seals are used) and the softness of the pads. The piston in the caliper generally has a square cut 'o' ring seal that actually twists when piston is being pushed out of the caliper, and when brake pedal is released, the square cut seal will untwist and pull the piston back slightly. This action is what gives the self-adjusting part some meaning.

 

During normal driving, once brakes are released, the moving rotor will wipe dust off the brake pad, giving it a very minor amount of clearance. When brakes are working correctly, pads will be just touching rotor, less than a thousandth of an inch clearance. With just the rotor spinning, it can never create clearance beyond the just touching point. Think about it, there is nothing there to make the clearance larger, it can only make the clearance to just touching. Similar idea to how a piston in an engine that is damaged with a piece of metal driven into the surface of the piston, can only drive the metal in to a point that it just touches the head to the point of zero clearance. It cannot possibly make more clearance(ignoring the expansion of metal from heat) due to it not moving into that area. Disc brakes work the same way. If you can spin the wheel (when off ground) and have it spin a minimum of a quarter turn after letting go, then it is good to go. Another test is to drive the vehicle normally for a couple of miles, and stop normally. Get out and feel the centre of the wheel on both sides. Heat should be near equal. A side effect of a sticking caliper is that the hotter it gets, the tighter it will grip the rotor.

 

When rotors were machined last year, did they measure them for thickness before machining? Rotors have a "machine to" spec and a "discard" spec. Generally there is 0.015 difference between the two numbers. Machine to number allows for normal rotor wear over the life of the brake pads. If a rotor becomes too thin, it will run hot, and will likely wear pads quicker due to the excessive heat. The extra heat will also impact caliper sliders and on drum brakes, hold down and return springs no rear drum brakes will lose their spring strength.

 

You should also consider changing brake fluid and flushing the entire brake hydraulic system to rid yourself of contaminated fluid.

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Lots of good information, I definitely appreciate it. I'm in the dark when it comes to how brakes and transmissions work for the most part. I had suspected caliper slide bolts but never thought that a rotor machined too thin could cause an issue. I'll be taking the truck in on Friday for the Stabilitrak issue and I'll have them look over the brakes to see if they can spot anything.

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