Rear disc brakes can be tricky
On The Road AgainJuly 25, 2012 | 3,402 views | Post a comment
Despite all the car components and systems that have been changed or improved with technology, the way brakes work has remained the same for decades. Anti-lock brakes are relatively new, but the basic friction principles and how they are attained is old technology.
Drum brakes, while being used less and less, are still around on lower end cars, and usually on the rear axle. These drums use a hydraulic wheel cylinder to spread out brake shoes, which jam into the drum, which slows the vehicle. Disc brakes are used on the front, and use a caliper to squeeze brake pads against a rotor for the same braking effect. Disc brakes are used more and more on the rear axle nowadays, and usually work better than the drum systems they replaced.
One thing has changed, however. When drum brakes are used on the rear, they generally last much longer than disc brakes do in the same position. Since 70 percent of braking occurs on the front axle, and 30 percent on the rear, it is unclear why this takes place, but I do have a theory. It seems that the much smaller pads on the rear are perhaps too small, and they wear prematurely.
I did a rear drum brake job recently on a ’89 Mazda truck that had 180,000 miles on it, and the rear brakes had never been touched until now. An ’05 Hyundai the next week needed rear disc brakes replaced at less than 50,000 miles, and the original front brakes still had well over 60 percent left.
It is noteworthy to mention that some rear calipers differ from the front when installing replacement pads during a brake job. On a disc brake, the caliper piston must be compressed to allow enough room for the new pads to be re-installed. This can be achieved by using a c-clamp on every front axle caliper I can think of. On some rear calipers, however, the piston is compressed back in by rotating the piston clockwise until it is retracted fully.
Recently, I ran across a caliper that I couldn’t figure out how to compress. I used Google to find out before an improper procedure ruined something. This particular caliper had to be rotated and squeezed simultaneously, which is a pretty tricky feat for those who don’t own every tool on the planet. Of course, they make such a tool to do both, and $60 later, its use was instrumental in the proper retraction and brake pad replacement.
So what did we learn from this? Firstly, when checking a car to see if it needs brakes, always pull a rear wheel as well as a front, as the rear can, and sometimes does, wear out first. Secondly, if you do your own work, most parts stores will rent this special tool for cheap, and you aren’t going to finish the job without it on certain models. And of course thirdly, you may want to purchase the silly thing and keep it around for next time. That way your neighbors can borrow it and you have an excuse to buy a new tool.