Brake vibration

[Greg Pace]

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.

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

A master cylinder from a turboboost model, the turboboost has a larger diameter cylinder, like a 1 1/8" diameter to a 1 3/16" if I remember correctly. When I replaced the Kelsey Hayes ABS unit, the vibration went away for a month, I thought it was fixed but it returned. I bled the brakes again, only right wheel had any air, fixed it for a week. I have bled the brakes about 10 times total, makes it a little better for a while. I have read that bleeding the brakes with a scan tool might work. I am looking for a scan tool with that capability, I will check the speed sensor data when I buy a new scan tool as you suggested, thanks

 

[Greg Pace]

How are your wheel bearings?

All new, front and rear. Thanks

 

[Erik the Awful]

Did you use the old school method of "tighten and then loosen two flats"? If so, it's wrong.

When I install bearings, if I can feel any play in the wheel I keep tightening. Typically they're all the way tight, loosened just enough to get a cotter pin in place. If I leave any more looseness I can measure the toe change.

 

[Schurkey]

A master cylinder from a turboboost model, the turboboost has a larger diameter cylinder, like a 1 1/8" diameter to a 1 3/16" if I remember correctly.

TURBOboost? You have a diesel master cylinder?

Or do you mean "Hydroboost"?

Either way, you probably have the WRONG master cylinder, incompatible with your front calipers.

A '96 C1500 almost certainly has JB5 brakes. If you ordered the heavy-duty brake package, you might have JB6 brakes. Either way, the front calipers, master cylinder, and vacuum booster are the same. The front calipers are "Low-Drag" for fuel economy. They require a 3-chamber master cylinder (Quick Take-Up) to work properly. I bet your "new" master cylinder does not have the third chamber.

That's not why your brakes are "shuddering", but it's still not right.

When I replaced the Kelsey Hayes ABS unit, the vibration went away for a month, I thought it was fixed but it returned. I bled the brakes again, only right wheel had any air, fixed it for a week. I have bled the brakes about 10 times total, makes it a little better for a while. I have read that bleeding the brakes with a scan tool might work. I am looking for a scan tool with that capability, I will check the speed sensor data when I buy a new scan tool as you suggested, thanks

Assure you have the right master cylinder. Bench-bleed the master, bleed the brakes to assure all the air is out of the master cylinder. Then auto-bleed the ABS, then bleed the brakes as usual using this .pdf guide.

See what happens.

 

[AuroraGirl]

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?

I mean more like play and looseness cause the rotor to pivot between the pads and the caliper which could mean wearing at changing angles or just wearing from being swiped periodically. That what you said tho, I could see it too. The hot rotor, theoretically, is going to be prone to deforming and doing that especially if its not the same exact thing happening to it at all moments. Do these trucks have a floating front caliper or is it fixed, fixed ones in this situation would be less prone to the problem i think but a floating may feel less harsh/drag more Id think

 

[Mikes98]

Does the pedal pulse and is the vibration worse when you lightly apply the brakes (as opposed to stomping on them for a rapid stop)? I am battling this situation; it happens most dramatically at higher speeds (55mph +) and in particular when I'm just lightly applying the brakes (to control speed and not necessarily stopping). When it first started I'd isolated it to the rear drums. When you have drums on the rear you can gently apply the emergency brake ...which only activates the rear brakes. The shudder would happen using the parking brake when gently slowing the truck from high speeds. You have to hold the release handle while you do this so the E-brake doesn't set and it's best to do this "test" when the road is empty or any following traffic is well behind you; you won't get any brake lights slowing the truck using the E-brake.

In my case it was the rear drums; one was quite out of round (.028, the other was .009). I believe you need no more than .005 out of round tolerance to get a smooth action from drum brakes. I installed new drums from NAPA; NAPA Premium drums ...made in China. Checking the new drums I had one at .009 and the other was .029 (you gotta love Chinese QC and apparently NAPA's lack of making sure their stuff is up to spec).
I returned that drum and the second one measured at .011. I didn't have the luxury of time; these drums were not readily available in NAPA's supply line. So I went with what I had. I should've had them turned but time wasn't on my side. Needless to say the shudder is still present and you can feel the pedal pulse although not as bad as before. So, the lesson is we are left dealing with sub-par new parts ...regardless the label on the box. Top on my list for the truck is getting the drums turned ...problem with that is finding a shop with a drum turning lathe that still works. My local machine shop doesn't turn rotors or drums because so many of the parts shops do it and he loses money on the service.