Brake vibration

[Greg Pace]

My 96 C1500 brakes shutter intermittently going down hills. No codes or ABS light. Replaced shoes, pads, rotors, drums, calipers, wheel cylinders (all GM new parts) and a rebuilt Kelsey Hayes controller, backing plates and speed sensors. It is 50 % better, buy not good going down a mountain sometimes (not always). I have a great pedal, I replaced the master with a boost cylinder. I'm wondering if the rubber front brake lines could do this, or maybe something else. I have bled the brakes at least 10 times. Anybody have experience with this problem?

 

[Nad_Yvalhosert]

Have you tried removing the ABS fuse, just to verify it's not ABS related?
ABS light off means the system is in good working order.

Have you tried turning the drums on a lathe? Darn near 90% of new drums are warped thanks to poor shipping conditions from China.

Hoses will either
A: restrict fluid and cause a pull
B: swell and cause a low spongy pedal
C: collapse and drag the pads, overheating and warping the rotors

 

[1998_K1500_Sub]

My 96 C1500 brakes shutter intermittently going down hills.

Did you measure the runout of the rotors after you mounted them? On both sides (inboard & outboard)?

From memory I believe max runout is .002. I’ve had new rotors measure <.002 (good!). I’ve had rotors that were turned on a brake lathe that were ~.010 (WTF)… tried re-clocking them on the hubs but they still measured almost exactly the same ~.010… seems the hubs were true, the rotors were not, so I assume the brake lathe or its operator weren’t up to standard.

Check the factory spec (don’t trust my memory) in the manual and measure the runout to see if yours meet spec.

Buy the most massive rotors you can. They’re not all the same, some have visually less mass than others, simply look at the vent’s air gap. I try to measure them on a scale when I can and I’ve seen some remarkable differences (although I don’t have my notes handy to quote them).

Finally, lesser quality rotors can have inhomogeneities in the makeup of the metal that may lead to unequal dimensional changes during heating (braking) and pulsation.

All of the above are factors that effect smooth braking under any conditions, but more noticeably under harsh conditions.

 

[GoToGuy]

At what conditions do you experience the shudder? (A shutter is on camera.) Do get feedback, pulsating brake pedal?
Only getting this during downhill driving, when the brakes are asked to do more than normal level ground braking.
I can get a chart that shows braking on downslope. Every degree down during braking , the inertia, and gravity work to substantially increase the actual weight of the vehicle. Requiring more braking power, higher load on brakes, higher brake pad, rotor temperature. Same with shoe and drum. Still a majority of braking energy is produced by front brakes. These temps are measured in Centigrade, not F.
A consideration is what type of pads or shoes are using? Organic, semi metallic, metallic, ceramic, carbon blend, carbon?
Could your driving style be a factor?
The question to answer first is it front rotors, or rear drums that are causing the shudder, or braking vibration. Good luck.

 

[Schurkey]

My 96 C1500 brakes shutter intermittently going down hills.

Braking vibration that comes and goes?

Two possibilities:
1. ABS engaging/disengaging

2. Minor vibration in two components that goes in and out of harmonic synchronization. You don't really notice either vibration until they both vibrate at the same time.

Of those two, ABS would be far more common. Connect a scan tool, have your partner look at wheel speed sensor data when going downhill.

Rotors and drums don't warp and un-warp.

I replaced the master with a boost cylinder.

What is a "boost cylinder"? Photos? Part numbers?

 

[1998_K1500_Sub]

Braking vibration that comes and goes?

Rotors and drums don't warp and un-warp.

I might believe your statement except I've had some personal observations over the years that taught me otherwise... that, or I've made many misinterpretations.

Most recently:

I was in Apr 2021 headed eastbound into Denver on I-70 where the elevation drops from 7700' (Genesse Park near MP253) to 5900' (at the US6 jct @ MP261), a 7.5mi stretch with mostly constant grade. I was in my 1998 Honda Accord, owned since 1999, with all new rotors I installed the year prior; the brakes had been smooth as glass. On the downhill descent it developed a very noticeable pulsation after extended application (and while disappointed, I wasn't surprised). Yes, I tried to use as much engine braking as possible but there comes a time when only the brakes will do.

After the brakes cooled (a few miles further along) the pulsation subsided but not completely. With additional use over time (i.e., the drive from Denver to northern IL) the brakes demonstrated reduced pulsation. Lately, the pulsation's only evident at a certain speed when, while braking, the pulsation resonates with the suspension (at maybe from 50-45MPH, from memory... I don't drive the car that often).

I've seen behavior like this on various vehicles over the years.

So when you said "rotors don't warp and un-warp" I raised an eyebrow.

 

[AuroraGirl]

Id think about a bearing being out of spec before thinking it warps and unwarps.

 

[1998_K1500_Sub]

Id think about a bearing being out of spec before thinking it warps and unwarps.

Interesting thought... the bearing / hub exposure to the heat from extended braking (via conduction from the rotor) would have increased the effective rotor runout and lead to pulsation? Did I get that right?

And that situation in the bearing would have persisted after cooling?

 

[Erik the Awful]

How are your wheel bearings?

 

[Greg Pace]

Have tried removing the ABS fuse, just to verify it's not ABS related?
ABS light off means the system is in good working order.

Have you tried turning the drums on a lathe? Darn near 90% of new drums are warped thanks to poor shipping conditions from China.

Hoses will either
A: restrict fluid and cause a pull
B: swell and cause a low spongy pedal
C: collapse and drag the pads, overheating and warping the rotors

I have removed the fuse, still does it.
I have turned the drums, replaced the drums, and turned the replaced drums, I have replaced the rotors 3 times, once from AutoZone, once from Napa, and finally a set of Delcos from Rock Auto, and I turned all three after the vibration was still there.