2007 Chevrolet Silverado Repair Question
Chevrolet Silverado Brakes Problem
The frames of the shoes can't move out far enough to catch on those six lands unless the drums were machined way beyond the published maximum limit or there were grooves already worn into the backing plate. While either of those are possible, it is more common to find a parking brake cable rusted in the partially-applied position or the shoes were over-adjusted. One problem with the parking brake cable theory is the truck is only five years old. The cables commonly rust tight in as little as one year, . . . on Fords; not on most other brands. Even if the shoes were manually over-adjusted, the linings would have worn down in short order and that would have been the end of that. To over-adjust more than that, it would have had to been done with the drums off, but then they wouldn't have been able to get them on.
I'd start by inspecting the backing plates for signs of grooves on the lands. They appear from the shoes sliding back and forth when the brakes are applied. I don't know what your shoes look like on the edges but GM used to use shoes that had just the edge of the frames riding on the backing plates. Almost everyone else has ears that are bent over to 90 degrees to provide a larger sliding surface so those grooves are much less likely to develop, and they pose less of a problem if they do. If there are grooves, they would not have affected the old shoes because for them to develop, the shoes had to have room to slide. With the new thicker linings, the frames have to be adjusted down to make room for them and that would put the frames back where they might catch and stay applied. I'd still want to see that for myself because once they got caught, the linings should have worn down quickly until there was some free play. Keep in mind too the drums are a larger diameter now from the machining process so the frames on the new shoes will be adjusted out a little further than where the old shoes started their life and where the grooves started to form.
If you do find grooves under the leading shoes, (the one toward the front of the truck), that would explain why both shoes wore down quickly. The leading shoes doesn't do much stopping. That's why it has a shorter lining. Its job is to grab onto the rotating drum and try to follow it around. That results in the bottom of it pushing the bottom of the trailing shoe into the drum. The piston in the back of the wheel cylinder pushes the top of the trailing shoe out, and it's that shoe that does the majority of the stopping. If the front shoe caught on a groove, both shoes would wear quickly. If the rear shoe was caught, only that one would wear. If both were wearing that hard, I would think you would have noticed the lack of power and sluggish acceleration.
I still find it hard to believe the shoes would wear that much to even be noticed so I wonder if the parking brake cables could be adjusted too tightly. The way to tell is to push the strut bar between the two shoes forward against the anti-rattle spring with your thumb. That bar should move a good 1/8", but less is acceptable as long as there is some free play. If it is tight against both shoes, it's too tight. Also look at where the two shoes contact the large anchor pin above the wheel cylinder. If either shoe is being held away from that pin, the parking brake cable is too tight or the shoe return springs are weak. They do get weak from the heat and are available in very inexpensive kits.
Also look for signs of leakage from the axle seals. Gear lube will cause chatter and grabbing and will deteriorate the linings. It can result in hot spots too on the drums.
Check if the shoes were installed correctly. The front one always has a shorter lining than the back one. If both are the same, they likely put the two leading shoes on one side and the two trailing shoes on the other side. I have to qualify that though for anyone else reading this. I'm pretty sure you have a "duo-servo" drum brake which just means the front shoe moves the star wheel adjuster to push on the rear shoe. A lot of front-wheel-drive cars have non-servo drum brakes. On those the leading shoe does 95 percent of the stopping going forward and the trailing shoe does most of the stopping going backward. Those leading and trailing shoes will be the same. In fact, they can be switched front-to-rear to get double the life out of the set. Those are identified by the fixed anchor on the bottom instead of a movable star wheel adjuster.
If they put the longer linings toward the front, they would grab the drums harder and apply more pressure than intended on the rear shoe. That higher pressure, along with the rear lining being shorter then designed, will cause it to wear too fast. The clue there is the front shoe would look like new and the rear one would be worn excessively. Some inexperienced people put the longer linings toward the front because they think those shoes do most of the stopping, which is incorrect.
If you do find grooves on the backing plates, we always coat those lands with high-temperature brake grease to reduce the chances of that happening. That is one of the many things do-it-yourselfers aren't aware of. If you see any of that grease, (it's typically black, silver, or copper-colored), you'll know the mechanic was conscientious and tried to do a good job.
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