Clunk when braking?

[ozymandias]

Hey all,

Background:

Recently acquired a Suburban (72 3/4 ton GMC w/ a 350 & 4spd manual) and have been slowly making it into a daily driver / work truck (with a lot of knowledge gained from posts here).

I've got quite a list of maintenance / upgrades to go through, but I've got a noise I can't pinpoint and want to get some feedback from everyone to see what the likely culprit is.

The problem:

The first time I step on the brakes, I hear a clunk that sounds like it's coming from the rear. The noise only happens once during a drive, but if I let it sit for a bit (anything over ~5-10 minutes), the noise will happen again (once) when I start driving and press the brakes.

What I've done:

Not much - mostly visual checks. I didn't see any leakage - especially from the brake drums or rear end, u-joints looked ok and didn't have too much play. I replaced the speedo cable and changed the transmission fluid.

Any ideas? All help is greatly appreciated!

[LockDoc]

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Did you check the center carrier bearing? The rubber sleeve may be bad or the bolts holding the bracket to the cross member may be loose.

Just some things to check.

LockDoc

[ozymandias]

Not yet, thanks for the tip!

I'll do that now

[vectorit]

This may seem like a lot of work, but if you haven't done it already then it might be worth doing.

It could be that your rear shoes are glazed and sticking to the drums, and the springs in the whole kit are weak.

If you've ruled out U joint's, and carrier bearing integrity. Then I'd rebuild the rear breaks, since it is not all that expensive to do if you do it yourself. Have the drums checked for true, and turned if needed. Then replace the shoes and all of the springs associated.

I've seen/heard drums make a clunk noise when the shoe assembly rocks forward and snaps back in the drum, this can be seen with evidence on the backing plate showing wear (shiny spots matching the shoes, including ridges for the shoes to stick on).

Other things to check before you dig into the brakes.
Check the shocks, and make sure they are not rotten. Rusty shaft, or just rotten. This is hard to tell on the truck, but if the shaft is rusty it's a good sign that the shocks should be replaced. Check the spring U bolts, and make sure the nuts are tight. If this is an original rig, this is probably not an issue. But if it has had the suspension messed with, it deserves to be checked.

[ozymandias]

Wow, awesome - thanks for the checklist Chris!

I'll report back tomorrow when I get some time under the truck. I am planning on upgrading the gas tank to the later model (bigger) one and will go through and double check all of the bearings / u joints & suspension components. If those are good, it's time for the brakes. The fact that it only occurs the first time they are used makes me suspect them more than the other components.

[ozymandias]

Quick update: I went through and looked at all of the various areas. U-joints & carrier bearing are in good shape, minimal play. One of the shock absorbers is junk - will definitely get that replaced next. The spring bolts are solid - she looks to be mostly original, and no one has messed with any suspension components from what I can tell.

Added to the to-do list:
Rear shocks
Rear brake rebuild