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Old 06-10-2005, 10:59 PM
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Warped Rotors

This is aquestion for the brake expertshere at HR.com.At work,I've got a 2000 Grand Caravan Sport that is making me crazy.Since new,this truck is warping front rotors way too often.It almost seems like any long distance,65-70 MPH run causes the rotors to warp.In 110000 miles,this truck has had 5 pairs of rotors put on,each due to severe vibration under braking.I've used both OEM and aftermarket rotors,replaced pads(min. wear),checkedcalipers,checkedfor binding caliper slides each time.
The rear drum brakes were just changed for the first time,but were checked whenever fronts were worked on .Operation of rear brakes has always appeared normal.I have ridden with the regular driver of this truck,and I don't believe that his driving habits re.use of brakes are causing problem.
Is there a chance that the proportioning valve is at fault?Truck has ABS and 17"wheels.Due to abs,I can't just do a hard stop on gravel to see if back wheels lock with fronts.Anybody have any ideas?
Thanks,George

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Old 06-10-2005, 11:09 PM
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Are you running the lug nuts on with an impact or are you using a torque wrench? What is the run out of the warped rotors? How many miles between cuts or replacement? This problem could be due to driving habits. Is this your van or a customer/friend? Do they live in an extremely wet area?

Steve
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Old 06-10-2005, 11:29 PM
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Wheels have been torqued to spec. in manual.
Runout has not been measured,but is enough to cause significant vibration under even normal-light braking.
Problem appears in as little as 5000 miles to as long as 20000 miles.
Truck belongs to my employer and is driven by a project manager.
I have ridden with him quite often,I don't believe he is causing problem-does't ride or abuse brakes.
This area is pretty average as far as wetness/rainfall-no flooding,etc.
Thanks,George
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Old 06-10-2005, 11:38 PM
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Have you measured the temps of the brake rotors after a normal drive? is 1 rotor warping more than the other? 5000 miles is a little soon but the 20,000 is not bad, most brakes last on average with normal every day driving aprox 20,000-25,000 miles or less. How about the flexible brake hoses, I have seen many collapse on the inside and not let the pressure off after the brakes have been applied. What type of brake pads are being used? they may not be venting off heat fast enough.

Steve
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Old 06-10-2005, 11:59 PM
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I haven't taken rotor temps.,but they do seem abit warmer thanthey should be.I have a non contact pyrometerto check temps.,what should range of temp. be?
With truck on lift and helper pressing and releasing brakes,all wheels apply and release in what appears to be the crrect manner.
Replacement pads have been std.type semi-metallic by either Wagner or Carquest.
Do you think a performance rotor like Powerstop or SSbc would last longer?
Std. type rotors have no crossdrilling or slotting,like a perf. rotor,and I wonder if that would help rotors last?
Thanks,George
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Old 06-11-2005, 12:04 AM
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How about wheel bearing? Are they in good shape? This can cause the rotors to run untrue of the calipers and cause overheating. I hope there would be other sings of a bad wheel bearing before it took out the rotors. You will want to look for a difference in temp from 1 rotor to the other. Is both front rotors warping or is it just one?

Steve
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Old 06-11-2005, 12:19 AM
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Wheel brgs. are nonadjustable,integral hub,with no signs of play or excessive drag.
Rotor on pass. side has been warped 3 of the 5 times,drivers side has warped twice.
Is there a spec or a way to test proportioning valve?Truck is usually lightly loaded,just driver and his tools and briefcase,and I'm wondering if prop. valve could be keeping rear line pressure too low.How can I check, other than changing prop. valve?
Thanks again,George
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Old 06-11-2005, 04:06 AM
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2 More cents

I woudl think that if every thing was normal and up to par so to speak the rears would be wearing at about the same rate as the fronts..At least on a normal brake job if the fronts are worn down to the point of needing replacemant the rears are not far behind and need replacement as well..

Leads me to believe the rears are not "sharing the load" with the fronts..So that leads me to believe the brake bias is not right front to rear..

I am not sure just how to check the propo valve on that but would ask if there is some way to disconnect the ABS just for testing and give it a try..Maybe the valve will just need to be replaced..

If you can check for any tech bulletins on that truck that may be a source of info..

Just my best guess..

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Old 06-11-2005, 09:35 AM
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The wheel bearings were a log shot, Are the rear brakes adjusted properly? I can assume that the rears are drum brakes since you said the drums were just replaced. Rear drum brakes should last 2 time as long (or longer) as the fronts if all is normal. Chrysler has had issues with there rear wheel cylinders going bad, usually they leak, but I have seen them fail to move. Do you have Alldata or any kind of shop manuals? I will be back at my shop on Monday morning. If you don't have service manuals I will look up the procedures for you. Again see if 1 rotor is hotter than the other after a drive. Don't over look the flex brake lines.

Steve
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Old 06-11-2005, 11:18 AM
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Did a bit of checking

I read my manual this AM and a defective master cylinder could cause this..One end is working just fine and the other end is not doing the job..may look OK on the rack but not doing the job on the street..

At least according to Ford manual this may be the case..Yours may be different...

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Old 06-11-2005, 11:56 AM
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I won't have access to truck until Monday,so I can only try to come up with a plan of attack until then.
I just ran a search of NHTSA service bulletins and didn't find any thing applicable,so I'm thinking I'll try the prop. valve and a thorough flush of the brake system. Before I do that I'll drive truck for a few miles and check temps of both discs and drums.I realize that I can't compare drum temps to disc temps,but this will give me something to compare before and after prop. valve change and flush.If I get no change,I'll try m/c.
I think you guys are right about prob being hydraulic-rear drum brakes shouldn't last 110000 miles while rotors are warping and runningwarm so frequently.The rear brakes have been showing only minimum wear at each time they've been checked,and the adjustmenthas had to be slightly backed off to remove drums,soI think they must be mechanically ok.
Truck was taken to dealer first 2 times vibration developed-rotors cut at 10000 miles and replaced at 14000-according to paperwork from dealer.Same tech did work each time-you would think he might've seen pattern developing.
Oh well-another challenge to be met and conquered.
Thanks to all for your help-I'llpost resultswhen I get this figured out.
Thanks again,George
Would there be any benefit if I put in an adj. prop valve instead of OEM replacement?

Last edited by Fast Orange; 06-11-2005 at 12:03 PM. Reason: add'l question
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Old 06-11-2005, 01:39 PM
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To test the valve you need the specs on it, and a gauge before and after the valve. You would be testing for what psi it begins to reduce pressure, and then the rate of reduction. These are called the 'split point' and 'slope'.

Look for a load sensing proportioning valve on the rear end, and make sure it is working if you have one. It reduces pressure depending on the height of the rod attached to the rear suspension.

Some vehicles no longer have a conventional prop valve, relying solely on the ABS to keep the rear brakes in check. It might still have a load sensing valve.

One thing to keep in mind is that runout in itself will not cause a pedal pulsation, but will cause brake surging on low speed stops. Driving with runout for extended periods will cause a thickness variation. Thickness variation is what causes pedal pulsation.
This is what I've experienced, and here are 2 articles which back it up, written by reputable sources:
http://www.stoptech.com/whitepapers/...otors_myth.htm
http://www.babcox.com/editorial/bf/bf100326.htm
They might help somewhat.
Always check runout on the vehicle after installing new or machined rotors.

It does sound to me like you're doing everything right. And the runout you are measuring each time is actually thickness variation which appears as runout on the gauge. Thickness vari can be caused by improper pad material transfer due to excessive heat.

More on rears...
This is a problem with some of the Dodge full size, and removing the load sensor is in a service bulletin... ONLY if the vehicle is always loaded.
People also like to add larger rear cylinders because it's the only thing that really works, esp if there is no load sensor. They have the same brakes as the Silverado, but the Dodge rear cylinders are half the size. This might not apply to the Caravan, but it's something to ponder on.

These Caravans were available I think with 2 different brakes. Hopefully you have the large ones.

The only thing is, if the rear brakes wear out in 40,000 on that, they're probably already doing as much as they can. If you have the smaller rears, maybe try to find the larger ones to improve your bias if it seems off.

Hope this helps.

edit: Just noticed the 110,000 mile rears. Dont know why I thought they got changed with the fronts a time or two...

Last edited by yesgo; 06-11-2005 at 01:45 PM.
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Old 06-16-2005, 05:18 PM
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I finally got a chance to work on this problem again.I took truck out for aride to get some heat in the brakes and see if any thing had changed.On a back road, away from other traffic,I did several relatively hard stops from about 50 mph,with only enough time between them to get back up to speed.After about the third or fourth such stop,I could smell the brakes getting hot,so I drove back to shop in a normal manner.To my surprize,by the time I used the brakes to turn into shop driveway,I noticed the vibration was much less than before the hard stops.At the shop,I checked the temps of both rotors and both drums with a non-contact pyrometer.Both rotors were close to 370* in center of pad contact area,both drums were about 180*,measured at out side center of shoe contact area.Something else I noticed was that outside edge of rotor/pad contact area was about 100* hotter than inner edge of contact area-maybe due to heat sink effect from hub/rotor assembly?
I let brakes cool for about an hour(until I no longer felt heat coming off them)and took truck back out .Vibration was still there ,but much less than start of first ride,so I repeated hard stops,but did several more than on first ride.Drove back to shop,parked truck for another cool off,and took truck out again.Vibration was now minimal.
Hopefully,the problemis the material transfer from pads to rotors talked about in links posted byYesgo (THANKS!).If vibration returns, I'll try a different type of pad material(ceramic maybe?)and see if that cures problem.
I think that by heating up brakes,they might have re-bedded themselves,and cleaned or evened out the transfered pad material.
What do you guys think? Am I out of the hole on this one or should i just put another set of pads on? Thanks for all you help,George
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Old 06-16-2005, 09:12 PM
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go w/ ceramics and make sure the caliper is sliding good check flex lines ive seen them where they would let fliud in but not out like a check valve.
good luck

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Old 06-19-2005, 08:45 PM
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I have been doing a lot of work with another auto manufacturer (my employer) on this issue. The base line problem is rotor run out after mounting on the hub. Measure the mounted rotor 1/4" from the outside edge for run out. Use appropriate mounting washers to torque the rotor into place. More than .002" run out will cause the rotor to wear unevenly, causing rotor thickness variation which causes the brake pulsation.

Debris between the hub and rotor, a brake lathe cutting run out into the rotor, a hub with run out issues, or improper torquing can contribute to the issue.

For additional details check out brake align. com.
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