Pulsating Brakes

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  #1  
Old 04-07-09, 05:21 PM
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Pulsating Brakes

Brought my 1990 Crown Vic wagon to a shop because whenever I stepped on the brakes there was a pulsation like intermittent braking. They replaced the front rotors and pads and the rear brake cylinders, which amounted to over $1000.
After a short while the pulsation returned. I checked and they did acually replace the rotors, pads and the rear cylinders.
Could it be that there is another cause than the brakes for this pulsation?
 
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Old 04-07-09, 05:42 PM
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The pulsation you feel is usually an OUT OF ROUND rotor.( It sounds better than Out of FLAT)

When you brake, the friction between pads and rotors are used to stop the vehicle. There is also a Tremendous amount of HEAT produced as a result.

Hard stops, prolonged application..(Driving Down hill while braking) or "Riding " the brakes can intensify the heat build up. The problem arises when the heat is not allowed to dissipate evenly. The remaining hot spots then "Distort" and you are left with high and low spots on a otherwise Flat surface.

Me personally.......After spending that much money with me, I would be happy to resurface the rotors for you. If that doesnt work, Pop em' off. Most Auto parts stores can resurf them for you for minimal cash.

If it makes you feel better......It happens...It isnt anyones Fault.
 
  #3  
Old 04-08-09, 01:30 PM
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Pulsation in the brake pedal is usually from "Thickness Variation" in the machined surface of the rotor. This thickness variation causes the caliper pistons to go in and out on each revolution of the rotor and that is fed back to the brake pedal as pulsation. Machining the rotors will only fix the problem for a short time unless you fix the cause of the thickness variation. If the rotor is not running true, each time the rotor turns the same part of the rotor scrubs the brake pads and sooner or later you have scrubbed of enough rotor material to cause thickness variation. Some causes of thickness variation are run-out on rotor to hub interface which could be rust or dirt or machined surfaces are distorted, uneven or incorrect wheel nut torque, hub mating surface not true, etc. General Motors dealerships have a very consise and evffective TSB for fixing this issue. If you can get your hands on a copy of it I am sure it will help you.
I was a chassis systems engineer for General Motors Corp Product Service Engineering for over 20 years.
 
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Old 04-08-09, 03:43 PM
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Originally Posted by hukre View Post
They replaced the front rotors ....
After a short while the pulsation returned.
How short of a while, after they replaced them? The replies so far act as if you never got new rotors. Is braking extremely hard and or long, in your repertoire?

Do you have to drive in SanFransisco down steep hills? Have you had previous similar brake problems in other vehicles or have your fellow neighbors or San Fansiscans?
 
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Old 04-08-09, 07:04 PM
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Thanks to all the people who took time to write a response to my post. To clarify: The reason I brought the car to a shop was the pulsating brakes, and on that basis they replaced the rotors, pads and rear cylinders. I realize that excessive use of the brakes may warp the rotors, but I don't think that I should have to resurface or replace them everytime after I make a trip to Tahoe (elevation 6500 ft) and back. Before I had the Crown Vic I had a 1968 Ford LTD and I never had any problem with the brakes, same for my Toyota pickup. It has been about 6 months since the installation of the new rotors and I made about 7000 miles. Could it be that the rotors are of inferior quality (they are made in China) ?
 
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Old 04-08-09, 07:12 PM
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As I said you most likely have run-out in the rotors that causes thickness variation. The usual mileage for a repeat probelm is 5000-7000 miles.
 
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Old 04-08-09, 07:23 PM
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Could it be that the rotors are of inferior quality (they are made in China) ?


There have been loads of problems with inferior quality parts,poor workmanship, poor quality metal,and the like with Imported parts...

Having said that, I wouldnt INSTANTLY blame a part for your troubles simply based on the "Made in China " label. Before coughing up the cash on new rotors, Resurface the ones you have, and try again. The Root cause may not be so obvious, and replacing them everytime you have a problem,is not cost effective.
 
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Old 04-08-09, 07:49 PM
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can also be caused by hard spots in the rotor. these can be either from original manufacturing defects or from improper break in of new rotors or not cleaning the anti rust coating from the rotors during installation process. it coats the new pads and then you have to use more brake force to stop thus overheating the rotors and causing hard spots in the rotor. unfortunately difficult to prove and hard to remedy sometimes resurfacing the rotors will cure sometimes not.

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  #9  
Old 04-22-09, 08:46 AM
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Best article I've ever found.Warped brake disc myth.

StopTech : Balanced Brake Upgrades

After years of frustration on a 1990 Honda Accord changing
rotors pads calipers and dealing with reoccurring pulsating
I've done this twice and cured it.Remove caliper from rotor
and hang out of way.Take Medium Grade Emery cloth and go
around rotor in circular motion a couple of inches at a time
(not like pads go around)like using a grinder.Do front and back
of rotor several times and break the pad glaze.If you have lots of
good pad left take them and put them on a vise facing up flat like
a tabletop.Take an all purpose file rounded on one side use round
side go across pad at 45 degree angle lightly in one direction then
the other until fresh material is showing then use flat side of
file to smooth out.Clean and regrease Caliper pins,etc.Re-assemble
and drive.At first there will still be a little shudder until shoes
reseat and clean up the rotor some more.

An exerpt from above article:

The obvious question now is "is there a "cure" for discs with uneven friction material deposits?" The answer is a conditional yes. If the vibration has just started, the chances are that the temperature has never reached the point where cementite begins to form. In this case, simply fitting a set of good "semi-metallic" pads and using them hard (after bedding) may well remove the deposits and restore the system to normal operation but with upgraded pads. If only a small amount of material has been transferred i.e. if the vibration is just starting, vigorous scrubbing with garnet paper may remove the deposit. As many deposits are not visible, scrub the entire friction surfaces thoroughly. Do not use regular sand paper or emery cloth as the aluminum oxide abrasive material will permeate the cast iron surface and make the condition worse. Do not bead blast or sand blast the discs for the same reason.
 
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