Brakes pulsating after front rotor/new pad replacement

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Schurkey

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Twenty-something years ago, there was a long and involved article making it's way around the Internet. Had to do with the "myth" of warped rotors, and how the real problem was pad material building-up on the rotor surface. "Rotors don't warp, they get contaminated with pad material".

Having cut hundreds of rotors, many of them due to customer complaints about pulsating brakes, and watching the cutter bit carve the high spots off the rotors, I never believed the story about pad material transferring to the rotor in any meaningful way. Certainly not enough to create high spots.

Trailblazer rotor, light silver is fresh metal as cut by brake lathe. Dark metal is worn metal from pad friction, rust pitting, and warpage. The dark metal parts are "low spots". Each successive cut reduces the amount of dark metal/low spots until the entire rotor surface is cut down to the level of the low spots. At that point, the rotor must be measured to assure it's still within service spec for thickness. It's entirely possible to cut the rotor so much that there isn't enough metal left for safety and heat absorption.
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My brake lathe is nearly as old as I am. It is not capable of providing a beautifully-smooth surface finish like you'd see with new rotors--or newer brake lathes. Even so, it's better than nothing. The Trailblazer rotors have six-ish months on them and working perfectly.

IF there's a rotor fault, I say it's either Runout, or Thickness Variation. Thickness variation is unlikely with a new rotor, entirely possible with a used rotor. Runout is a possibility with a rotor of any age including new-in-the-box; because we all know who took a giant, steaming-hot dump in the Replacement Brake Rotor Well.

Checking for runout on a new K2500 rotor "riveted" to a 180K hub via the lug-studs. (Result was approx. .001 Total Indicated Runout (TIR))
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454cid

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My brake lathe is nearly as old as I am. It is not capable of providing a beautifully-smooth surface finish like you'd see with new rotors--or newer brake lathes. Even so, it's better than nothing.

I keep looking at them when they come up for auction, but I don't really have the room for one. I'd love to be able to turn my rotors and drums.
 

HotWheelsBurban

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I used to have a drum/rotor lathe, from the parts store. I was the one who did most of the work with it, and 98% of the time, no problems. But occasionally I got some drums and rotors in that had glazing so bad, the only way to rectify it w was to hand cut them in reverse of normal operation. Took me a while to learn how to do this, but the few times I had to use the technique, it worked.
I used the lathe on our own vehicles, once we closed the store. It was an Ammco model 3000, brand new in 1983 but not the most deluxe model. Had to sell it a few years ago, to my friend with the hot rod shop.
 

tayto

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i'm not really sure that a low spot is going to cause pulsation. without seeing the corresponding rotor face the only problem I see here is reduced braking.

you can stick your nose up to bedding in brakes, that's fine. it is an important thing for a race car or your daily driver. i have seen and felt the difference, it works. i do 4-8 brake jobs per year out of my home garage. i use quality pads & rotors and i bed them in. no one has come back or complained about pulsating and i live in the Rockies. fully loaded trucks/vans in mountainous terrain really shows how well your brakes are working. as for modern pads that don't need to be bedded in, are you talking about ceramics? i still bed them in but it is more to heat cycle the rotor than material transfer.
 

Schurkey

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Don't think that I'm against bedding-in pads. I do that with every brake-job.

I just don't see pad material building-up on the iron of the rotor face in a way that causes pulsation. When I feel pulsation, the brake lathe shows warpage as it cuts.
 

Schurkey

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i'm not really sure that a low spot is going to cause pulsation. without seeing the corresponding rotor face the only problem I see here is reduced braking.
The low spots create reduced pad pressure, and therefore reduced friction. As the rotor turns while braking, it goes from "full braking" to "reduced braking" and causes the car to shudder as it stops. You may/may not feel this in the brake pedal; you're very likely to feel it in the steering wheel and seat, as one side of the car brakes strongly while the other side doesn't--and then the reversal of strong/weak braking forces as the rotors turn.

The "other side" of the rotor may be the opposite of the side shown--where the one face has a low spot, the other face may have a somewhat-matching high spot. That's not guaranteed, it's not like you'd have a mirror-image. Warping and wear can create unique high/low spots on both rotor faces.

Given that I don't have time constraints; if I see a rotor that seems to be overly-symmetrical in the warpage when I start to cut the thing, I'll loosen it on the lathe axle and reposition it 90 degrees around. This assures that I don't have the rotor mounted improperly--the mounting hung-up on an iron chip, or something. The rotor has to be mounted "true" to the axle that's turning it on the lathe; and that axle can't be bent or riding on failed bearings.
 

tayto

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Don't think that I'm against bedding-in pads. I do that with every brake-job.

I just don't see pad material building-up on the iron of the rotor face in a way that causes pulsation. When I feel pulsation, the brake lathe shows warpage as it cuts.
a few years back my fathers pilot needed some brake work. we didn't have all the parts at the time so put new calipers on all corners. the rear rotors were rusty and pitted so had a friend turn them on his brake lathe at home. old pads were almost at end of their life so we threw them on to get him home (did not bed in). while at home he had a local guy throw some new raybestos pads on. all was good for a few months and then the pulsating started. got all the brake parts in, i assumed fronts were the issue. new premium rotors, hawk lts pads + bedded in. braking performance hugely increased but pulsating still there. next visit same treatment on the rears, no pulsating anymore.

now, The rotors that got turned were still in spec thickness wise so what was it? heat? high spot from pad material? turned in correctly? he tows a trailer in the mountains so very likely they got hot...
 

thinger2

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before you put new pads in you need file the burrs off of the backing frame of the pad.
They are not a bolt in part anymore.
You have to hand fit the pads to the slides .
They can bind and get partially stuck etc..
All of that has to work properly before you do anything else.
Have you replaced the caliper brake rubber hoses?
They rot inside the hose and cause most of all front brake problems.
And it is always an intermitant problem when it first appears.
Try new front brake lines.
If you havent replaced them.
It needs them.
 
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