Da Groj

I suppose there are other forums to ask this type of question, but I'd rather just ask you guys if you don't mind.

I replaced the pads on my car as well as my wife's car this week.

My wife's CX9 has had the same brakes for 85,000 miles.
The rotors looked fine to my untrained eyes. They are smooth on both sides. I did not replace them, or turn them.
To my surprise the pads appeared to still have about 40% remaining. However, the vehicle used to vibrate a bit upon braking. After I replaced the pads, it does not vibrate anymore.

The brakes seem really good now, but I have a couple of questions. Does it matter that I did not replace the rotors? I actually bought new rotors, but didn't feel like it was necessary to replace them. Does it matter that I did not bleed the brakes? Also, when I pushed the brake piston back in to make room for the new brakes, I pushed them almost all the way in, so when I was done, the brake pedal went to the floor. I pumped it a few times, and it returned to normal. Again, I hope the brake fluid is unaffected by this.



My own car is in a slightly different situation. I replaced all the pads on it, but the front rotors were not quite as smooth as they could be. I could barely feel some rounded grooves in them. I couldn't see them, but again, I could barely feel them. Also, the brake pads were absolutely SHOT. In fact, the brakes were squeaking when I went in reverse, giving me the warning that they were due. I also had a brake warning light illuminated on my gauge cluster. It's gone now.

I have had a set of Hawk HP Plus brake pads in my garage for a couple of years. I only used them for one day at Sebring, so they are good. However, I do believe that this type of pad will finish off my rotors because it creates more friction, right? Should I let these pads finish off my rotors, assuming they will? I drive my car very little, so it will take a while.

One more thing about my rear brakes. When I twisted the rear piston clockwise to sink it into the caliper, it twisted one of the boots significantly. The boot is still tight. It just looks less than clean. Is that a worry?

Obviously, I'm a total noob with anything mechanical, so I really appreciate any advice.
Chuck

Chuck, I'm no mechanic, but I've always done my own brake jobs.

In reference to your wife's car, I think you should be ok. There are a number of things that can cause vibration under braking, including bad tie rod ends, etc... If the rotors aren't grooved or worn (usually you can feel a lip on the area outside the contact patch with the pad) you are probably ok for DD duty. Especially if changing the pads seemed to remedy the vibration feedback.

In regards to brake fluid, I would have probably bled them, but I think you can get by if the peddle still feels good. It's normal to have a soft peddle at first after pushing the piston back in for new pads. At 85k, the fluid is probably a little dirty, but if you didn't open the system to introduce air in the lines it should be safe to drive.

I recently ran a set of HAWK HPS+ pads at Sebring in April. I went through the fronts in a weekend (8 sessions) plus driving to and from Sebring, the rears were about 50-60% worn. They were extremely harsh on my rotors, but that was track duty. They are a bit noisy and dusty, but can be used on street/auto-x as well. The idea is that you can drive to the track with them and they work well on the street/auto-x because of their operating temp. If you aren't getting horrible vibration and feedback from the current rotors, you could stick it out in the sake of saving $. Then you can do the new rotors and new pads at the same time.

I really liked the Hawk HPS+ as I feel they are a versatile pad, but I wouldn't DD on them (which it sounds like you are not doing).

Your wife's rotors are fine, don't sweat it. The vibration actually WAS probably mild deposits on the rotors, and the process of bedding in the new pads took care of it.

Regarding race pads (or "aggressive" pads) and rotor wear, brake pads are designed to primarily work by "ablation" of the pad material. If you have pads that are well-suited to the operating temperatures that they see, some pad material initially transfers to the surface of the rotor, and then it's the interaction of that layer of pad material on the rotor with the pad itself that gives you the uber-friction to stop the car. In a perfect world, the rotors should wear very little.

But, here's what happens in the real world... with standard street pads, the operating temp is low enough that the pads work in ablative mode like they're supposed to most of the time, but the world is still imperfect. Every time it rains, your rotors rust up a little bit, and you end up losing your transfer layer and scrubbing off some rotor material until the pad rebeds itself. If you let that rust go for a couple days, it's even worse. And if it's on the rear of a car that doesn't get driven hard, sometimes the pads never actually rebed without some assistance. (I used to drive the snot out of my wife's car and drag the handbrake for a bit about once every month just to make sure the brakes stayed healthy) Also, in the real world, we do drive when it's cold... and the brakes work in "abrasive" mode until they reach operating temperature. And, of course, we also sometimes overheat the brakes, which can cause problems, as well. (such as leaving "scars" of the brake pad imprint from the hot brake pad being left against the hot rotor when you park the car)

Race pads do all of the same things, except they work at a higher temperature. The more "racey" the pad, the higher the temp range, and the harder it is to get them up to operating temp. That simply means that they work in "abrasive" mode anytime they're not hot.

I remember putting a set of Cobalt SM (Spec Miata pads) on my Miata the day before going to Sebring once. I drove around a little bit that evening thinking I might bed in the pads... I could see the sparks flying off of the front rotors from the driver's seat! The pads did nothing but grind away the rotors in street use. But, after half a lap and a couple good hard uses on the track, they were up to temp and WOW did they work!

Regarding brake fluid and the phenomenon of the pedal going to the floor after pad replacement, that is normal. Your first press of the pedal simply took up the free space between the pads and rotors. (I've heard that more than one garage door has been damaged from newbs not realizing that this happens)

Flush your brake fluid! Even on your wife's car, it's not good for more than 2-3 years. Oh, sure, it will "work", but it has absorbed moisture and its boiling point has reduced by A LOT (likely from 400+ degrees to well below 300). So, the brakes will be fine for general use, but when she makes that one "panic stop" and generates some heat, the fluid will instantly boil, pedal feel will go away... and at a minimum, dear wife will freak out about it. (worst case, she goes to make a second hard stop and ends up with nothing)

Good article on brake rotors and how pads work:
http://www.stoptech.com/technical-suppo ... ther-myths

Also more good brake tech articles:
http://www.stoptech.com/technical-suppo ... ite-papers

Oh, lest I forget...

When you replaced the pads, did you take the rotors (somewhat) apart and clean and lubricate the slide pins? A lot of people don't know to do that. And when you're talking about brakes that don't get attention more than every 60-80k miles (like the rear brakes on most cars), those slide pins will lose their lube and eventually get some moisture in there and begin rusting. Then you end up with sticky and eventually frozen calipers.

If you didn't do it, it's worth the effort to take the wheels back off and do it. Will ensure proper brake function AND save you money. (when the caliber freezes up due to the rust, you'll have to replace it!)

Because the front brake pads usually get replaced at least twice as often as the rears, I usually clean and lube the rear brake rotors anytime I do the front brakes.

Thank you both for the replies. I appreciate the time you took to respond. It gives me some peace of mind.

It looks like I have some work to do! I greased the slide pins on my wife's car, but not on mine. Grease came with her pads, but not with mine.
I will also replace the brake fluid. I replaced the fluid on my car a couple of years ago with some type of high performance fluid, but it's probably time again.

Question about the rear brakes. I used that special tool to turn the rear piston clockwise to push the piston back in. Is it acceptable to turn that piston counter clockwise, to untwist the rubber dust boot around it? And then turn it back again?

Rosko wrote:Question about the rear brakes. I used that special tool to turn the rear piston clockwise to push the piston back in. Is it acceptable to turn that piston counter clockwise, to untwist the rubber dust boot around it? And then turn it back again?

Yes.

Heh. Just reread my posts above. Forgive all the typos. Geez!