Single rear rotor replacement, now some lockup

[dquarasr2]

Pertinent info:
- Brembo OEM-equivalent replacement rotors all 'round with under 1000 street miles and about four track events
- Cobalt GT-Spec pads

I noticed a small crack in the left rear rotor, so I ordered another OEM replacement from Tire Rack. I just replaced it.

I did not need to turn the piston (I don't have the tool) to re-install the caliper, but I did ensure that the nub on the inboard pad aligned with the slotted "x" in the piston.

The caliper was relatively tight when it came off, and was similarly tight but seemingly no more so when I re-installed it. I did need to tap it in place with a hammer to install the floating caliper bolts, but when reassembled I was able to turn the rotor (with the wheel attached, gave me more leverage) by hand fairly easily.

The rotor I removed did not seem to have much wear on it at all. There is really no noticeable ridge where the contact area ends, so I'm guessing (I don't have a measuring tool) it's very close to the new one. I also surmise that since reinstalling the caliper over the pads was pretty easy, the new rotor is not significantly thicker than the cracked one.

After I put the thing back together I took the car out to bed in the new caliper. The first stop gave a big lockup, after which the ABS pump clicked itself to life. Subsequent stops were OK, but I was getting what seems to be premature ABS engagement, pretty much exclusively from the left-rear, the rotor I replaced. It didn't feel like I was getting maximum braking. I did notice, though, that of the dozen or so stops I did it seemed to have been getting better a little at a time.

Questions: Do you think this will go away as the rotor wears in and beds with the pads? - or -

Did I miss some sort of adjustment? Should I go out and get the piston tool and start over, pushing the piston in per my search results on Prime and my review of the shop manual? Remember, I did not have any trouble clearing the pads when I re-installed the caliper, and it was only slightly tighter when I was done.

Thanks as always to any replies!

 

[L8apex32]

I claim to know a little about brakes... but nothing is real obvious that I see.

Here are some thoughts that came to mind, however.

1) Why/how can you crack a rear rotor? The fronts do the majority of the braking. I've never heard of rear brake problems like cracked rotor or them locking up. Something seems wrong here.

2) Can you tell if the other rear brake is working? Sure you can tell. Do a few stops to get things hot, then go see if both rear rotors are hot. An infrared (touchless) pyrometer would be nice here.

3) Make sure to bleed your brakes. At least, make sure all four corners have been bled to remove any air or other bubbles in the caliper. A few squirts at each corner may really help.

4) Check the fronts also. If they aren't working for some reason, then the rears will have to do more work which may show off a rear brake problem. Check the front rotor temps also.

5) Do you have the same pad material front and back? Make sure the front and rear brakes are bed in with the same material and the same amount of bedding in. If you wanted to go more down this route, you could get all for rotors turned - or just sand them down. I've heard you need to use a special sand paper so look for additional advice here.

6) You might just want to start by getting the cheap rear caliper tool and screwing in the piston a few turns (so the caliper goes on easily). This will help ensure even grip in back.

Hope this helps.

 

[dquarasr2]

I claim to know a little about brakes... but nothing is real obvious that I see.

Here are some thoughts that came to mind, however.

1) Why/how can you crack a rear rotor? The fronts do the majority of the braking. I've never heard of rear brake problems like cracked rotor or them locking up. Something seems wrong here.

2) Can you tell if the other rear brake is working? Sure you can tell. Do a few stops to get things hot, then go see if both rear rotors are hot. An infrared (touchless) pyrometer would be nice here.

3) Make sure to bleed your brakes. At least, make sure all four corners have been bled to remove any air or other bubbles in the caliper. A few squirts at each corner may really help.

4) Check the fronts also. If they aren't working for some reason, then the rears will have to do more work which may show off a rear brake problem. Check the front rotor temps also.

5) Do you have the same pad material front and back? Make sure the front and rear brakes are bed in with the same material and the same amount of bedding in. If you wanted to go more down this route, you could get all for rotors turned - or just sand them down. I've heard you need to use a special sand paper so look for additional advice here.

6) You might just want to start by getting the cheap rear caliper tool and screwing in the piston a few turns (so the caliper goes on easily). This will help ensure even grip in back.

Hope this helps.

Thanks for your comments.

1. I was a little surprised by the rotor cracking. It was not cracked all the way through, i.e., it was a radial crack, but funny, it was only in the center of the pad contact area. The contact area closest to the hub was not cracked, and the contact area near the outer edge of the hub was not cracked. I could not see if the crack went all the way through the outboard side (it's a vented rotor, so I could not see the "inner" vent side it), but I thought it prudent to change it considering I understand when rotors do fail they can do so fairly catastrophically. I don't want to be heading into a turn on a track and have a failure!

2. Both sides were functioning.

3. I didn't bleed the brakes but then again I didn't think it was necessary since the brakes were behaving before I changed just the one rotor. The car will need a fluid change before next track event, so that's something I'll do.

4. Fronts don't seem to be a problem. ref: everything was behaving before I changed the one rotor.

5. Pads are the same Cobalt GT-Spec all around. Tire Rack provides the rotors with the cross-grind pattern out-of-the-box. I did notice that toward the end of the dozen or so stops I did it was starting to behave better, and the pads were forming their own grooves in the rotor.

6. I think I'll just go ahead and do this first chance I get. I do know that I've had some touchiness with my Miata (locking up the fronts if the rear park brake adjustment was per shop manual, that is, if backed off 1/3 turn after contact - I needed to adjust them snug so they're close to the pad to avoid the fronts locking up).

(One other thing that comes to mind. I didn't have any brake cleaning fluid, so I wiped the rotor down as best I could and slapped it on. I wonder if there wasn't some sort of anti-rusting coating that causes it to grip until it's burned off.)

Again, thanks for your comments. I am hoping that I was premature in being concerned, and that simply driving it and bedding them in will remedy this situation. I know that I have had inconsistent braking when installing new pads, until everything settled in, so maybe I was simply paranoid in this situation. However, it is brakes we're talking about. :biggrin:

 

[NSXGMS]

Pertinent info:
- Brembo OEM-equivalent replacement rotors all 'round with under 1000 street miles and about four track events
- Cobalt GT-Spec pads

I noticed a small crack in the left rear rotor, so I ordered another OEM replacement from Tire Rack. I just replaced it.

I did not need to turn the piston (I don't have the tool) to re-install the caliper, but I did ensure that the nub on the inboard pad aligned with the slotted "x" in the piston.

The caliper was relatively tight when it came off, and was similarly tight but seemingly no more so when I re-installed it. I did need to tap it in place with a hammer to install the floating caliper bolts, but when reassembled I was able to turn the rotor (with the wheel attached, gave me more leverage) by hand fairly easily.

The rotor I removed did not seem to have much wear on it at all. There is really no noticeable ridge where the contact area ends, so I'm guessing (I don't have a measuring tool) it's very close to the new one. I also surmise that since reinstalling the caliper over the pads was pretty easy, the new rotor is not significantly thicker than the cracked one.

After I put the thing back together I took the car out to bed in the new caliper. The first stop gave a big lockup, after which the ABS pump clicked itself to life. Subsequent stops were OK, but I was getting what seems to be premature ABS engagement, pretty much exclusively from the left-rear, the rotor I replaced. It didn't feel like I was getting maximum braking. I did notice, though, that of the dozen or so stops I did it seemed to have been getting better a little at a time.

Questions: Do you think this will go away as the rotor wears in and beds with the pads? - or -

Did I miss some sort of adjustment? Should I go out and get the piston tool and start over, pushing the piston in per my search results on Prime and my review of the shop manual? Remember, I did not have any trouble clearing the pads when I re-installed the caliper, and it was only slightly tighter when I was done.

Thanks as always to any replies!

Are the rear rotors drilled? I have had rear Powerslot drilled rotors crack after only 5000 miles (200 track). Never again will I use drilled rotors. If the holes are cast into the rotor that's different but they still pose a greater cracking risk than non-drilled. Plus, there are virtually no performance benefits.

Back to your issue: the obvious factor to me is that it had something to do with the piston not being screwed back in. I would remove the caliper and do it properly. Let's eliminate that as the source of the problem.

The fact that you could get the caliper off at all without screwing the piston in is curious. Unless you're He-Man. :wink: It's possible the piston is no bueno and the caliper needs a rebuild but I would do the brake job properly first and see if that doesn't solve it. If it was working fine before the install it should be fine after the install.

You can rent the caliper tool at Autobone for free or if you're resourceful a wide-bladed chisel and a pair of vice-grips will suffice.

Also, a fluid bleed and good fluid like Motul or ATE is highly recommended. You might want to clean out the piston/boot area thoroughly with brake cleaner and re-lube it with the proper lube that won't corrode the rubber boot. Check for any tears/holes in the boot.

In addition, it's recommended you follow these steps to properly bed the brakes:

For a typical performance brake system using street-performance pads, a series of ten partial braking events, from 60mph down to 10mph, will typically raise the temperature of the brake components sufficiently to be considered one bed-in set. Each of the ten partial braking events should achieve moderate-to-high deceleration (about 80 to 90% of the deceleration required to lock up the brakes and/or to engage the ABS), and they should be made one after the other, without allowing the brakes to cool in between.

 

[dquarasr2]

Are the rear rotors drilled? I have had rear Powerslot drilled rotors crack after only 5000 miles (200 track). Never again will I use drilled rotors. If the holes are cast into the rotor that's different but they still pose a greater cracking risk than non-drilled. Plus, there are virtually no performance benefits.

Back to your issue: the obvious factor to me is that it had something to do with the piston not being screwed back in. I would remove the caliper and do it properly. Let's eliminate that as the source of the problem.

The fact that you could get the caliper off at all without screwing the piston in is curious. Unless you're He-Man. :wink: It's possible the piston is no bueno and the caliper needs a rebuild but I would do the brake job properly first and see if that doesn't solve it. If it was working fine before the install it should be fine after the install.

You can rent the caliper tool at Autobone for free or if you're resourceful a wide-bladed chisel and a pair of vice-grips will suffice.

Also, a fluid bleed and good fluid like Motul or ATE is highly recommended. You might want to clean out the piston/boot area thoroughly with brake cleaner and re-lube it with the proper lube that won't corrode the rubber boot. Check for any tears/holes in the boot.

None of the rotors are drilled or slotted. I think that simply invites cracking.

I don't understand when you replied "The fact that you could get the caliper off at all without screwing the piston in is curious. Unless you're He-Man."

How can the piston be screwed before it's removed? I assume you mean "could get the caliper ON at all. . ." If that's what you meant, it was a little tight but not enough that I could not tap it in with a hammer. The new rotor was probably *just* a bit wider than the cracked one that had some miles on it.

All pistons were rebuilt about 18 months ago, and it gets fresh Motul 600 about every three or four months so it's always good for the next track event.

I think it is a good idea to disassemble and properly retract the piston, clean using brake clean, and lube as you suggest. I'll do that in a few weeks when I have a free weekend (coming weekends are all full!)

Thanks for your comments.

 

[NSXGMS]

None of the rotors are drilled or slotted. I think that simply invites cracking.

I don't understand when you replied "The fact that you could get the caliper off at all without screwing the piston in is curious. Unless you're He-Man."

How can the piston be screwed before it's removed? I assume you mean "could get the caliper ON at all. . ." If that's what you meant, it was a little tight but not enough that I could not tap it in with a hammer. The new rotor was probably *just* a bit wider than the cracked one that had some miles on it.

All pistons were rebuilt about 18 months ago, and it gets fresh Motul 600 about every three or four months so it's always good for the next track event.

I think it is a good idea to disassemble and properly retract the piston, clean using brake clean, and lube as you suggest. I'll do that in a few weeks when I have a free weekend (coming weekends are all full!)

Thanks for your comments.

Correct, I meant on. :redface:

If the pads were low and the piston had moved outward all the way due to low pad material it's possible it's stuck one way or another. Jamming the caliper on with a new pad might have jammed it......

Good to know you're regular w/ the fluid flushes and use good fluid. It's the blood of the brake system and is generally changed not often enough.

 

[nsxtasy]

I've used solid-faced rotors, drilled rotors, and slotted rotors for my often-tracked NSX, and I keep track of how long they last, in terms of total miles and track miles. There has been absolutely no significant difference in the lifespan of the three types of rotors. Yes, when the drilled rotors crack, the cracks start at the drill holes, but they don't do so any quicker than with the solid-faced or slotted rotors.

And rear rotors last, well, almost forever. I've never had them crack. I just replaced a pair that got thinner than spec, but I think they had 8,000+ actual track miles on them.

BTW, Cobalt sells GT Sport pads (old formulation was a street-track pad, the new ones are a bit more aggressive), and they sold various track-only pads under names like Spec VR, Spec B, Spec C, etc. None are/were called GT Spec.

 

[dquarasr2]

I've used solid-faced rotors, drilled rotors, and slotted rotors for my often-tracked NSX, and I keep track of how long they last, in terms of total miles and track miles. There has been absolutely no significant difference in the lifespan of the three types of rotors. Yes, when the drilled rotors crack, the cracks start at the drill holes, but they don't do so any quicker than with the solid-faced or slotted rotors.

And rear rotors last, well, almost forever. I've never had them crack. I just replaced a pair that got thinner than spec, but I think they had 8,000+ actual track miles on them.

BTW, Cobalt sells GT Sport pads (old formulation was a street-track pad, the new ones are a bit more aggressive), and they sold various track-only pads under names like Spec VR, Spec B, Spec C, etc. None are/were called GT Spec.

Well, so far in my experience with this car, it had slotted rotors when I first got it, and the fronts cracked starting at one of the slots, and now I've had this rear rotor crack. I think it might have something to do with my technique, which I've since modified, causing the brakes to get hotter than they otherwise would. I had a tendency to ease into them too much, in a misguided attempt to transfer weight to the nose, however, I was a little too slow before applying full braking. What I was told is that this actually gets them hotter in the long run. I did try getting to maximum braking earlier in the braking zones, and much to my surprise, ended up with managing the brake temps better.

What did I write? GT-Spec? Yes, I meant (previous generation) GT-Sport. I have Cobalt Spec Miata pads on the other car, goofed when I typed this up.

Thanks for your reply.

 

[L8apex32]

And rear rotors last, well, almost forever. I've never had them crack. I just replaced a pair that got thinner than spec, but I think they had 8,000+ actual track miles on them.

Read this and remember folks. Racing teams pay big bucks to run endurance tests on their equipment to find out actually how long stuff lasts. This is very valuable information.

I might add that in my similar experience, my rears have never cracked. The cracked rear rotor in this thread may point to a stuck rear caliper piston or parking brake mechanism.

 

[dquarasr2]

Read this and remember folks. Racing teams pay big bucks to run endurance tests on their equipment to find out actually how long stuff lasts. This is very valuable information.

I might add that in my similar experience, my rears have never cracked. The cracked rear rotor in this thread may point to a stuck rear caliper piston or parking brake mechanism.

Yes, the unusual nature of the rotor having cracked to start with is not lost on me. Before I track it again I'll either go high-tech and purchase a laser-pyrometer or at least go low-tech and apply some temperature-sensing paint. Better safe than sorry when it comes to brakes.