Brake Problem has me Stumped....

The vehicle: 1999 Chevrolet Suburban 2500.  Just purchased a couple of weeks ago.  157,000 miles, 350 Vortec gas engine, 4x4

The issue: Under moderate to hard braking, the brake pedal pulses.  The pulsing feels like an on-off-on-off type of feel, and the vehicle lurches forward and backwards as it's stopping.  Sometimes, depending on how hard I'm pushing the brakes, the steering wheel will jerk back and forth at the same time as the pedal is pulsing.

This does have the Hydroboost system (hydraulic power boosted brakes, runs off the power steering pump) and I have tried the bleeding procedure (turn wheel full lock both directions several times) and that hasn't affected the problem.  The pulsing does seem speed-dependent, that is, it gets slower as the vehicle slows down.  It feels for all the world to me like warped rotors, but I pulled the front wheels last night and pads and rotors look fine.  I spun the rotors by hand and they spun true, with no discernable wobble.  Pads look about 1/2 worn.  

I tried applying just the parking brake at speed, which would engage just the rear wheels, and those seemed to work OK, no pulsing, so I'm inclined to think the rear drums are fine.  

When not braking, power steering works fine at all speeds.  

The truck also has ABS, but this doesn't feel like an ABS pulsing; it's slower and, as I said, more speed-dependent.  

Thoughts?  

Broken belt in a tire, bad wheel bearing or front end component. Thise are my wag.

Uneven transfer layer on the front rotors causes this, as the pads go past the sticky spot.  New or turned rotors are the solution, and you probably should put better quality pads on than the PO did to try to avoid this in the future.  I’m really pleased with the Raybestos specialty truck pads on my 03 Sub 2500, very reasonably priced on Rock Auto. 

Dusterbd13-michael said:

Broken belt in a tire, bad wheel bearing or front end component. Thise are my wag.

Tires look almost new (P/O told me they had 5k miles on them, and they're good truck tries), wheel bearings seem fine (checked the play when I had it jacked up) and front end links are greased and don't appear to have excessive wear.  Truck overall appears well cared for.  

One thing I did notice is the front anti-sway bar bushings are shot, and I do plan to replace those.  I don't believe those would contribute to this issue, though.  

Rotors don't actually warp. They get uneven material transfer as sonic pointed out.  Not someth8ng you can really measure.  Sometimes you can't see it either.

New pads and rotors will clear that up..

Sonic said:

Uneven transfer layer on the front rotors causes this, as the pads go past the sticky spot.  New or turned rotors are the solution, and you probably should put better quality pads on than the PO did to try to avoid this in the future.  I’m really pleased with the Raybestos specialty truck pads on my 03 Sub 2500, very reasonably priced on Rock Auto. 

This makes sense, never heard of that before though.  In my head and to my finely tuned mechanic's sense, it feels for all the world like warped rotors, but I hate just shotgunning parts.  Especially since replacing front rotors on a 4x4 is a non-trivial job (or at least, less trivial than on a 2 WD model).

I, too, like the Raybestos pads.  

wvumtnbkr said:

Rotors don't actually warp. They get uneven material transfer as sonic pointed out.  Not someth8ng you can really measure.  Sometimes you can't see it either.

Huh.  File that under "T-I-L".    Thanks.  

This deal looks pretty good.  Full brake package all-in-one, and I can pick it up today (and thus not wait on Rock-Auto shipping).

Anyone have any experience with NAPA's house brand brake stuff?  

You might try to re-bed them. 2-3 hard stops from 60 mph to 20 mph to get them good and hot. Then park it and let them cool. 

Ive used the brown scotchbrite pads on a die grinder to "resurface" rotors before. May be worth a try to see if the behavior changes before a non-trivial brake job. Do both sides of both front rotors.

Toyman01 said:

You might try to re-bed them. 2-3 hard stops from 60 mph to 20 mph to get them good and hot. Then park it and let them cool. 

This is what I would try first.  

I did try re-bedding them last night, after checking them.  Went out on the road late at night and made some hard stops.  No appreciable change in behavior.  

Depending on the length of time the vehicle was sitting before you purchased it, the rotors may have started to rust. The area covered by the pads did not. This will cause the condition you are describing. We used to have this condition at new car dealership when the cars would sit on the lot for a long time. We would remove caliper and sand/buff both sides of the rotor and pads. Reassemble and road test. 

Had a braking wobble like that on our older odyssey. Replaced front rotors and pads. Only got slightly better. Rotated tires front to rear. Almost completely fixed. 

I have no explanation for why it was only felt under braking, but it did seem to be a tire balance issue. 

Since the truck has 4WD, I'd love to jack the whole thing up, pull the wheels, and put it in gear and sand down the front rotors.  Problem is the dust shields make sanding the back half of the rotor difficult.  If I'm going to go through the trouble of taking the rotors off, I'll just replace them, since I have to pull the hubs to do that.  

In reply to volvoclearinghouse :

If you pull the front calipers, you should be able to do the back of the rotors. On a GM that should only be two bolts. 

Toyman01 said:

In reply to volvoclearinghouse :

If you pull the front calipers, you should be able to do the back of the rotors. On a GM that should only be two bolts. 

 

Just thought of that, too.  Thanks!  

The trouble is that the transfer layer goes a bit deeper than the surface for sanding, and will come back if the rotors aren’t cut or replaced.  I went through this on my in laws car, tried all of those sorts of things, worked ok short term but came back, the solution was new rotors and better quality pads 

Have the rotors turned and I can almost guarantee you will be good to go again. I have seen rotors out of the box needing turned. 

I have seen a rotor rust thought and separate the disc from the hub on one of these suburbans.

Not mine but another GMT400 that belonged to a co-worker. Had an intermittant functioning front brake on on side that felt like what you are describing. Sometimes the disc would jam on the hub and you would have a working brake, sometimes the hub would spin inside the rust out rotor and you would have no brake.

Professor_Brap said:

Have the rotors turned and I can almost guarantee you will be good to go again. I have seen rotors out of the box needing turned. 

I second the motion.   This is the first thing to do. And probably cheapest.  

With this front end you need to find a shop with a Snap-On Kwik Lathe that cuts the rotor on the truck. Not many shops have or do this, but it saves a lot of labor.

A couple stops is not bedding the rotors. At leas 6 50 to 10mph slowdowns (dont stop completely or you just made it worse!) Until you smell hot brakes and your pedal gets soft. Then bleed them and you are good to go. 

In reply to volvoclearinghouse :

I've used NAPA brake stuff several times.   No complaints.

If it were me, I would pull the ABS fuse and test again.  One of the sensors might be freaking out the pump, and 1999 GM ABS was iffy technology.  I would want to rule it out, since it's so easy to do.