Brake shudder - please help?

[sadsack]

Vehicle: 2003 Subaru Legacy wagon. F/R disk brakes.

 

Problem:

 

Brake vibration/shudder during moderate braking after brakes have warmed up. Under heavy braking, vibration disappears. Vibration is felt primarily through the steering wheel, and is proportional to speed.

 

History:

 

Bought new - I'm the original owner. 34K miles, driven hard, but well maintained. No curbs or Auto-X (hah! in a Legacy wagon?).

 

Brake fluid changed at 30K service.

 

Previous trip to dealer about this problem resulted in them resurfacing the rear (!?) rotors for excessive runout. They checked the fronts but left them alone.

 

Front pads 50% worn, rears also have 50% wear (?!). Car is still holding a factory spec alignment.

 

Lug nut torque has been periodically checked, by me, with a torque wrench, so that they do not exceed 75 lb/ft. I had a Ford econobox before, so I know about the consequences of improper/uneven lugnut torque.

 

 

I'm worried about three things:

 

1) My 36K/3yr bumper-to-bumper warranty runs out in 2K more miles, so I need to resolve this quickly.

 

2) Rear pad wear seems high - I would think that the fronts would be eaten up much more quickly relative to rears.

 

3) Can this be caused by the tandem brake booster? I find it unusual that the vibration goes away when I press hard on the brake pedal. There's also the mysteriously high wear on the rear pads.

 

I'd appreciate any good ammunition for my next encounter with dealer service. I'm reasonably competent with DIY, I just don't have a runout gauge, and would rather the dealer fix this on their dime instead of having to fix it myself at this point in time (after 36K, sure).

 

Thanks!

[tunered]

this is more than likely rotor warpage,at slow speed you dont really notice it unless it is real bad,under heavy braking it acts the same. this is not a major job to do your self but im not sure about being covered.

[Suzam]

Sounds like you may have some pad material transfered onto the front rotors due to heat. You could ask the dealer to relace the rotor, but they will most likely want to resurface them, which would relive the symptoms also.

 

There are several links about the same problem you can try a search. One discussion was about replacing the pads with better than OEM material.

[tcspeer]

I think the rear pads or smaller to start with, so they may not be wearing that much more than the front. I agree with what the others say you probably need new rotors on the front. If you can afford it replace with new instead of turning.

Vehicle: 2003 Subaru Legacy wagon. F/R disk brakes.

 

Problem:

 

Brake vibration/shudder during moderate braking after brakes have warmed up. Under heavy braking, vibration disappears. Vibration is felt primarily through the steering wheel, and is proportional to speed.

 

History:

 

Bought new - I'm the original owner. 34K miles, driven hard, but well maintained. No curbs or Auto-X (hah! in a Legacy wagon?).

 

Brake fluid changed at 30K service.

 

Previous trip to dealer about this problem resulted in them resurfacing the rear (!?) rotors for excessive runout. They checked the fronts but left them alone.

 

Front pads 50% worn, rears also have 50% wear (?!). Car is still holding a factory spec alignment.

 

Lug nut torque has been periodically checked, by me, with a torque wrench, so that they do not exceed 75 lb/ft. I had a Ford econobox before, so I know about the consequences of improper/uneven lugnut torque.

 

 

I'm worried about three things:

 

1) My 36K/3yr bumper-to-bumper warranty runs out in 2K more miles, so I need to resolve this quickly.

 

2) Rear pad wear seems high - I would think that the fronts would be eaten up much more quickly relative to rears.

 

3) Can this be caused by the tandem brake booster? I find it unusual that the vibration goes away when I press hard on the brake pedal. There's also the mysteriously high wear on the rear pads.

 

I'd appreciate any good ammunition for my next encounter with dealer service. I'm reasonably competent with DIY, I just don't have a runout gauge, and would rather the dealer fix this on their dime instead of having to fix it myself at this point in time (after 36K, sure).

 

Thanks!

[1 Lucky Texan]

pretty much what they said. I have had bad tires behave kinda like that on braking once. More likely rotor issues.

 

I learned a lot from this site;

 

http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

[sadsack]

Thanks for all the replies so far.

 

I initially suspected warped rotors, but hell, it's still under warranty - why get dirty?

 

I had a Ford Escort, so I've got plenty of experience diagnosing warped rotors and seized caliper sliders. The situation with the Scoob is somewhat different, in that:

 

1) If it's warped rotors, then I should notice pulsation/vibration whenever I use the brakes. In this case, however, hard braking yields no vibration, while light and moderate braking causes vibration.

 

2) Why would the rear brake pads be worn to the same degree (50%) as the front pads (assuming the dealer is accurate/honest in the wear estimate)? From what I understand, the fronts account for 70-80% of the stopping power. If anything, the fronts should have crapped out before the rears (which were just turned for excessive runout), unless the stock braking system is "perfectly" balanced wrt pad wear?

 

3) The fact that pulsation/vibration is only present during light/moderate braking leads me to suspect a possible problem with the tandem brake booster. As noted above and in tunered's reply, I would expect warpage to cause vibration whenever brakes are applied. Could the "dual-stage" nature of the vibration possibly be attributed to a malfunction of "wimp" stage (low boost) of the booster? I've lurked for a while and have read the threads regarding poor pad life (i.e. "You drive like a maniac"), but have not seen any description of the particular behavior I've noticed.

 

As far as PROPER repairs go - once the brakes are out of warranty and it's my dime, I'll definitely replace pads and rotors all around, add SS lines, clean & lube calipers, slides, and bushings, and do the StopTech bedding procedure .

 

Until then, I'd like to have as persuasive an argument as possible to convince the dealer to fix this problem on THEIR dime.

 

Please keep the suggestions coming - I'm very grateful.

[Setright]

Rear pad wear:

 

A 2003 model is likely to have EBD. Electronic Brakeforce Distribution will make the rear brakes work harder in a straight line and ease off them in bends. "Accelerated" wear is a common complaint. At work, my boss' Toyota needed new rear pads before fronts - at 30k miles!!

 

Shudder:

 

The most likely cause is rotors. If the pad deposits are still very thin then hard braking can "mask" the shudder.

You'll need to be very persuasive to get the dealer to fit new rotors and pads. Perhaps they can meet you half-way if you supply the pads? That way you can better pads than those stock "Jurid" pads.

[sadsack]

Setright -

 

I scoped the shop manual, and the only thing modulating the rears (under non-ABS operation) is the proportioning valve. Under hard breaking the PV acts to reduces fluid pressure to the rears, so I assume that hard driving will actually increase the pad wear difference between front and rear (with the fronts taking a greater hit). Then again, I may ASSuming too much.

 

Pad deposits sound plausible, though, as they would essentially act like the tines in a music box, causing brake pad flutter under light breaking. If this is the case, I might as well get a free turning to last another 5-10K before I swap the whole kit. Once that cementite is seeded in the pads - you're using the lathe as a lawnmower, when you really need Round-Up . . .

 

Thanks again!

[1 Lucky Texan]

Setright -

 

I scoped the shop manual, and the only thing modulating the rears (under non-ABS operation) is the proportioning valve. Under hard breaking the PV acts to reduces fluid pressure to the rears, so I assume that hard driving will actually increase the pad wear difference between front and rear (with the fronts taking a greater hit). Then again, I may ASSuming too much.

 

Pad deposits sound plausible, though, as they would essentially act like the tines in a music box, causing brake pad flutter under light breaking. If this is the case, I might as well get a free turning to last another 5-10K before I swap the whole kit. Once that cementite is seeded in the pads - you're using the lathe as a lawnmower, when you really need Round-Up . . .

 

Thanks again!

 

Let us know how it works out.

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