chuck's

My Suburban has a noticable pulsation in the rear brakes when the pedal is depressed hard.You can feel it even with a normal application but not as noticable.I have eliminated the fronts because when I use the parking brake pedal and apply the rear brakes, the pulsation appears.When it first showed up I took the drums in to get machined.The drums were worn beyond their limit so I bought new ones from Napa.I thought it fixed the problem,but it came right back.I got another set of drums from Napa,changed the shoes,ensured the adjusters worked and tried again.Same pulsation.I would like to eliminate this problem.I would appreciate any assistance in solving this problem.Thanks in advance.
chuck's

GMMerlin

If you are 100% sure the pulsation is coming from the back, then what is happening is you have some excessive drum runout.
Typical causes are:
1. improperly adjusted rear shoes...there should be about .020 to .030 clearance between the drum and the shoe..if the shoe is contacting the drum while driving, the drum will heat up
2. heavy application of the parking brake especially when the drums are hot..in most situations, the parking brake does not need to be used..off the top of my head I can only think of 3 times I have applied the parking brake on my Sub.
Check the hardware, if it is sticking the shoes may not release properly.

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chuck's

Thanks for the response.I may be focused only on the drums because I'm not convinced that the drums manufactured in China have the same quality assurrance as in North America.I did check pricing for GM drums and it was astronomical.Purchasing from Napa or Lordco will get you cheaper drums but they do claim just as good as OE.I did check the hardware and assurred that the adjusters were functional.I even left the shoes out of adjustment and allowed the adjusters to get to the proper setting.I could hear the adjusters click with each brake application and then it finally stopped.I have been asked if my axle is bent?I feel that if it was bent,I would feel some vibration when driving.The vehicle rides very smoothly for its age.So I don't think that the axle is bent.I have talked to my local brake shop and they are willing to check the run out on the brake lathe.I think that this will tell me how much run out I have.What is an acceptable amount of run out??I want to thank you again for your help.
chuck's

GMMerlin

The aftermarket drums could be your problem also...
Maximiun runout should be no more than .006"
You dont need to check it on a brake lathe, a drum micrometer will do just as well.

__________________
Why do you never have enough time or money to do the job right the first time, but enough time and money to do it again?
Got all the certifications
25 years ASE Certified Master Technician
GM WCT
Yea..I work at a DEALERSHIP....got a problem with that?

Want to diagnose cars like the Pros? FOR SALE: OTC Perception 2 channel Labscope (badged as a Matco Reality)
This is my personal labscope that I use to diagnose waveforms, like new, comes with leads and training video
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j cAT

since you replaced the drums and shoes did you adjust the parking brake cable.......?

you did have no pulsations at first running on replacements,,, then the pulsations returned....


the cables for proper adjustment and binding....lube the interior park brake mechanism as well....

if the front brakes need work/bleeding worn/rusted rotors or inferior pads the rear shoes do more braking than the spec...this is also a reason for drum distortions..

chuck's

I have gotten the drums turned.Right now it seems to be much better.I have had just enough time to get the drums on and drive.I will do a follow up as I get the time.
chuck's

I did do a check with the park brake pedal to check for pulsation.It doesn't have the same pulsation as before.I was asked if I the pulsation was coming from the front.Since I wasn't feeling a pulsation from the rear when the park pedal was being applied,I decided to change the front rotors.After a road test with the new rotors installed,it appears that the pulsation disappeared.If there is any change over a period of driving,I will update.
chuck's