Pulsing feeling when braking

[alaskanjer]

I’m having a braking issue that I’m hoping for some help diagnosing. When I apply the brakes moderately hard at 30-40 mph, I get a pulsing feeling as if there were a warped rotor or uneven friction material transfer. I don’t feel it in the brake pedal at all or the steering wheel in those circumstances (I do feel it in the steering wheel in light braking at hwy speeds). It feels as if the brakes have more power at one half of the wheel rotation than the other.  It doesn’t pull to one side or the other. It doesn’t feel anything like ABS (way slower, no sound, can’t feel it in the pedal.) It happens with equal intensity on my summer wheels and my winter wheels w/studs.

 

The car is a ’98 Outback with 248,000 miles. I have checked everything I can think of with the brakes and I can’t find anything wrong. 

 

The rear pads and rotors have 70,000 miles on them. Pads are about 75% gone. The rear rotors (brembo OE) are at 50% (9.25mm, down from 10mm new.) I measured runout yesterday and it is .001 on one side and .0015 on the other. I measured two places, near the top and near the bottom of the brake track. Front pads/rotors have 40K on them. Pads look great for the miles, definitely more than 50% remaining. Rotor (Napa Ultra Premium) is less than 24mm but greater than 23.5mm (24mm new.) I measured runout in three different revolutions on the front rotors (top, middle, bottom of brake track.) Left was .001, .001, and 0., right was .001, .0005, and .001. Pads are Napa “Adaptive One” at all four corners.

 

Last time I bled the brakes was two years ago when I did the front pads/rotors and replaced a frozen front caliper. This problem started showing up about 6 months after that and I thought it would probably be a caliper seal from the (poorly) remanufactured caliper. There were a couple of minor pits in the pistons and I figured they were weeping brake fluid and contaminating the pads. However I couldn’t find any evidence of leaking or contamination on the pads.

 

So I don’t know what to check next, but pretty much everything in the suspension and steering is original. 

 

[Rooster2]

I would suspect just a minimal amount of warp to your front calipers. It can be amazing how so little warpage can be the source of the brake pulsing felt in the steering wheel. Most likely problem is with the front rotors, since you can feel it in the steering.

[1 Lucky Texan]

swap the tires front to back first. probably not an issue but - easy to try. And sounds like you feel certain there's no binding of caliper movement.(some people remove the rubber bushing on the slider pin)

do a bedding-in procedure. a moderate one first. If no change at all (better OR worse) do an aggressive bed-in . If the problem gets worse after any bed in, you may have a part of a rotor with altered alloy characteristics (cementite ?) and you will need a new rotor.

there are different ways to do bedding in, just make CERTAIN you do it in an area where it
will be safe (from other people and police) to accelerate and slow down aggressively - followed by a cool-down run. You MUST NOT stop and hold the pads against a hot rotor.

 

might be worth inspecting the brake caliper bracket and bolts - in the remote chance something is cracked.

Edited June 10, 2015 by 1 Lucky Texan

[Fairtax4me]

Did you measure the width of the rotors in several places for thickness variation?

[lstevens76]

Have you tried bleeding the brakes?  Sometimes air in the system can mess with the ABS and cause a pulsing sensation.  The same thing applies for a vehicle speed sensor used by the ABS.  If it is starting to fail it could be throwing some erroneous information that is causing the issue.

[forester2002s]

+1 for warped front rotors.

If this was my car, I would install new front rotors and brake pads (they're due anyway).

[alaskanjer]

I would suspect just a minimal amount of warp to your front calipers. It can be amazing how so little warpage can be the source of the brake pulsing felt in the steering wheel. Most likely problem is with the front rotors, since you can feel it in the steering.

 

is the way I'm measuring runout, except I am measuring the runout at a few different diameters, not just near the top of the brake track like he does in the video. Both front rotors have 1 thou or less lateral runout. I believe 1 thou to be within spec. 

 

 

swap the tires front to back first. probably not an issue but - easy to try. And sounds like you feel certain there's no binding of caliper movement.(some people remove the rubber bushing on the slider pin)

 

do a bedding-in procedure. a moderate one first. If no change at all (better OR worse) do an aggressive bed-in . If the problem gets worse after any bed in, you may have a part of a rotor with altered alloy characteristics (cementite ?) and you will need a new rotor.

 

there are different ways to do bedding in, just make CERTAIN you do it in an area where it

will be safe (from other people and police) to accelerate and slow down aggressively - followed by a cool-down run. You MUST NOT stop and hold the pads against a hot rotor.

 

might be worth inspecting the brake caliper bracket and bolts - in the remote chance something is cracked.

 

I have smooth movement of the caliper bracket back and forth so I have been spared the rubber bushing/slider pin gremlins that some folks have had. Also, with pads removed and partner gently pressing brake pedal both pistons of both front calipers seem to move pretty much in unison. (as opposed to one of them binding and the other doing all the movement.)

 

I can't entirely follow you suggestion to do a moderate bed in first because I already did an aggressive one a couple days ago. And it definitely didn't make the pulsing go away, but I didn't think about that making a possible cementite issue worse. Good idea, thanks for that suggestion. I will take it for another spin and see if things got worse since I did that.

 

I did swap wheels around and couldn't tell any difference. I swapped front to back and also swapped between my summer and winter wheels/tires.

 

I should also mention that I am the only one who has changed wheels on this car since way before the brake issue started and I ALWAYS use a torque wrench (75 ft. lbs.) and I tighten the lug nuts in the normal star pattern.

 

Did you measure the width of the rotors in several places for thickness variation?

 

I did not. Thanks, another simple check i should have done.

 

Have you tried bleeding the brakes?  Sometimes air in the system can mess with the ABS and cause a pulsing sensation.  The same thing applies for a vehicle speed sensor used by the ABS.  If it is starting to fail it could be throwing some erroneous information that is causing the issue.

 

I haven't re-bled yet. They were last bled 3/3/12. I didn't think it could be ABS because it is so much slower than (properly functioning) ABS, (about once per wheel revolution vs. 60 times per second,) and the more the car slows down, the slower the pulsing gets. Also, I can't hear it or feel it in the brake pedal, and I can when the ABS kicks on when I am on ice or in a panic stop.