|
If the hubs are anything more than warm after a good run then the pads are dragging and the calipers needs attention. Rears usually start to drag more often than the fronts, so if it's hot fronts you know things have been neglected. Pad dragging is almost always caused by the piston failing to adequately retract, and to a far lesser extent the pad edges possibly binding in their slots due to rust. Some of the retraction is the extended rubber seal pulling back the piston a tiny bit, which lessens as the seal edges wear. Most of the retraction is the pads being pushed back by rotor movement and expanded hot rotor metal. This will simply keep increasing until the wheel is one step short of frozen. All the while your gas mileage is suffering and your rotors and pads are wearing far faster than they should.
What's happening is that as the piston seals wear, minute traces of brake fluid on the piston are in essence oozing out past the seals and getting on the exposed end of the piston. Under heat, the fluid dries and cakes on the pistons to a brown residue that makes it all the more difficult for the piston to retract in through the seal in addition to seal wear. The exposed brake fluid will also attract any dirt making the dried residue all the worse. If the piston dust seals are damaged, both brake dust and water can get in and allow the piston to rust, which is game over for that piston. If grit gets in it can lead to scoring, again game over for the piston. Rust can also be caused by any hydroscopic moisture in the brake fluid.
For most people, it's caliper replacement time. Finding reliably good reconditioned calipers is a whole other story.
For people like me, as long as the pistons remain pristeen and I'm fairly sure everything except the rubber is in good shape then I rebuild my own calipers. I've done at least a dozen on my Volvos. Assorted seal kits with new rubber seals, dust boots, guide pin boots and bleeder nipples are available. They used to be cheap, but they're now halfway to the price of a new caliper. Dealing with worn guide pins and worn guide pin bushings in the caliper mounting frames is another consideration. At that stage it's no longer an easy DIY fix. Caliper rebuilders vary on properly replacing the guide pin bushings. I believe few do this, most will replace only worn pins, which are available. Over the years occasionally replacing guide pins I've come to learn that guide pin bushing wear is equal to pin wear, so new pins only addresses half the problem.
As a bit if a trip down memory lane, I'll mention that if you don't have the funds for new calipers or the time and skill to rebuild them yourself then you can try extending the life for another year or so by removing the dust boots, extending the pistons as far out as you can without popping out and cleaning off the dried piston residue with brake fluid and a toothbrush. Rinse with spray brake cleaner, push the pistons back in all the way and repeat until there is no more residue on the pistons and they easily retract. The dust boots on the front single piston calipers cannot be removed so cleaning options are limited to trying to get brake cleaner under the lip, basically not worth the effort. For the rears, the caliper should be removed for piston access and great care taken not to pop the pistons out or puncture the dust boots. In my starving student days I used to do that, but never since then once I learned to rebuild them without risk of mistakes.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
|
