Rear drum brakes grabbing

[noelb]

1998 Burb with rear drum brakes. Seems in wet weather the rear brakes seem to grab. Last night driving in the rain I noticed a very distinct difference in how the brakes were working. Sometimes like normal and then sometines very touchy and only as light pedal pressure required with the back brakes threatening to lock up.

This morning as I was driving down our steepish drive way slowly the rear brakes were grabbing anf locking on and off with hammering sound, wheels sort of pulsating as when I looked, as I was braking to a stop. Though maybe the Linings were grabbing and bouncing up and down.

I though it may have been adjustment andhave adjusted them up now about 15 notches.

Won't know whether that was the problem until it rains again.

Has any one else had a similar problems with the back brakes grabbing and locking up? Any solutions?

[DmaxMaverick]

Not good. It usually means a seal leak, and soaked shoes. Could be either brake fluid or gear lube. If they are out of adjustment and grab, they should grab all the time. Better have a look inside.

[noelb]

I wouldn't expect to find any problems inside as the entire unit hub to hub has only about 20,000 kms on it but, I'll have a gander. No external signs of leakage. I only have problems in the wet weather.

[Bnave95]

My truck had done this all the time after driving in rain and sat over night. Did a shoe rebuild using Auto Zone Dura brand,does not do this any more. Our GM van also did this. Once shoes were replaced all was fine.
I have heard on this forum, many having the same issue.

[DmaxMaverick]

When you did your last shoe change, do you recall if there were primary/secondary shoes? If so, one will have a longer lining than the other. If these are installed inverse, it could cause grabbing. I've ran into this on other models, but don't recall if your rear end had that option.

[wfyehl]

My Sub had done the same thing, also. I looked closely at the shoes, and even though the shoes had plenty of material on them, they were bad. When attempting to clean them with brake parts cleaner, the shoes started to crumble apart. I put new shoes on it from AutoZone and haven't had any problems since. I don't know what causes the shoes to dry up and crumble (excessive heat).

Bill

[gmctd]

The rear brakes have an automatic wear adjuster mechanism which functions only when you're backing up - seems as tho just backing outta the driveway is not enough to keep up with normal wear.

Symptom is grabby rear brakes - defeats the proportiong valve normal f\r bias calibration

So..........you'll need to get up under there and do the job your self - just pop out the covers in the slots, jack up the rear wheels, turn the star-adjuster while rotating the wheel till the brake shoes firmly stop the wheel, then back off till the wheel rotates with only minor shoe\drum contact.

Purpose of forcing firm contact is to center the shoes into the drum diameter, such that the release is concentric, or exactly parallel to the drum braking surface.

Try it - you'll like it.................

[DennisG01]

Could the pulsating be your anti-lock brakes kicking in? Obviously they shouldn't be under the cicumstances you mention, but the way you described the pulsating sounds just like anti-locks. If it is the anti-locks, you should also feel the pulsating in the pedal.

[noelb]

23,000 kms on brand new drum to drum diff assembly. Haven't checked inside drums but I have adjusted the brakes manually. I have tried to get the brakes to adjust by reversing and braking but that doesn't work. Seems the adjuster is too stiff perhaps. I will see how it goes now they have been adjusted up.

Thanks for all the info. It will give me something to work on.

By the way the GM dealerships over here don't seem to have much Idea. They say that some blokes have spent Hundreds (likely thousands) of dollars on this issue with drum machining etc. Seems it could have been just adjustment or the OEM linings all along. You really do have to keep GM accountable when you take your truck there!

[JohnC]

Every GM truck I've had did this until I replaced the shoes. Regardless of the mechanism, it has a lot to do with the shoes.

The self adjusters work very slowly over time, so a single or even several applications in reverse may not have a noticable effect.

[G. Gearloose]

Yup, first see if the adjuster is frozen, loose brakes grab

Also file a bevel on the leading edge of each shoe, it keep the first-application locks away.

[Mainous]

This problem is created by improper adjustment of the rear brakes. The method suggested above is not correct either. Per the GM service manual, adjust until the drum can barely be turned by hand, then back off 33 notches on the star adjuster.
I always tighten up the adjuster as indicated then mark the adjuster with white nail polish. The adjuster has 26 notches so I back it off one full turn plus 7 notches more. You have to use a long small screwdriver to push the adjustment mechanism away from the adjuster.
These are duo servo brakes and they have to have room to rotate in the drum. If they don't, they grab as indicated in the post.
I have seen this screwed up by more than one GM service department. Make sure they read the manual.

[gmctd]

Good post -

Might be a lot safer for 2500HD and 3500 vehicles with 12-13" brakes, incl full-floating rear axles, to use the first method - they are not equipped with duo-servo brakes.
The adjustment indicated is about 20 clicks, but - I'll admit to it - I do it by 'sense' for best feel.

The second method is most correct for 1500 and 2500 vehicles with standard brakes.

You may notice, tho, that both methods indicate 'loading' the shoes into the drum before adjusting for release.

Indicates that merely taking up the slack is not commensurate with 'driveability', eh?

[Mainous]

gmctd,

Are you saying that C/K 2500 w/8600 gvw and C/K 3500 don't have Duo-Servo brakes?

[noelb]

This problem is created by improper adjustment of the rear brakes. The method suggested above is not correct either. Per the GM service manual, adjust until the drum can barely be turned by hand, then back off 33 notches on the star adjuster.

I ham having touble getting my headarround this. All the vehicles I have ever had to adjust drum brakes on Has always been to adjust up until you can't turn the drum and back off 3-6 notches.

If my burb had Duo servo brakes then they were adjusted up too much already - only 15 notches back - Is that right

When I first adjusted the brakes I adjusted them about 30 or so notches until the drum would not turn BECAUSE this brought the brake pedal up. It seemed to go a long way down under heavy braking.

[Mainous]

noelb,

I think you just found your problem. Follow my procedure and you will be fine. By reading your signature your truck definitely has duo-servo brakes.
The pedal indeed would have felt taller the way you adjusted them but only a portion of each shoe is biting. The way these things work is that the turning drum literally rotates the shoe assembly wedging it into the drum. The more force applied, the more braking you get. If you don't back them off the 33 notches they don't have enough space to rotate.

[gmctd]

6600gvwr and above do not use the duo-servo scheme - that's some 2" x 12", and the 2.5" and 3.5" x 13" brakes, and specifically not, in the large hydroboost systems.

[noelb]

GMCTD I do have the Hydroboost. What does that mean for me?:confused:

Do I back off 33 notches? My old Diff has 13 inch drums with 3.5 inch wide linings. I haven't checked the one on the car.

8600 lbs rating with RPO C6P & JD7

[Mainous]

gmctd,

I'm not sure where you are getting your information but 6600 gvw and above are specifically what the duo servo's were designed for and all C/K2500 & 3500 with drum brakes that I'm aware of have them, with or without hydroboost.

[gmctd]

As you may know, all 6.5's get the hydroboost systems, 1500 to 3500, C and K - if you do not have full floating rear axle, and are below 6600gvwr, then you more than likely have the duo-servo braking system.

[Mainous]

noelb,

Send me your email address and I will send you a scan of the proper pages from the GM manual.

mainous@cinci.rr.com

I'm not sure that we are all on the same page as to what duo-servo means but I assure you, your Suburban has duo-servo brakes.

[noelb]

Thanks Jeff, Can't find the procedure in The online manual, I suppose because they are self adjusting and therefore no procedure necessary.

Email address on the way

By the way I don't know which page I am on because the page numbers came off when my wife put them through the wash - That's my story and I'm sticking to it.

Cheers

Noel

[gmctd]

Hmmmm....looks like my bad! :o

I have been unconciously describing duo-servo action, in centering the shoes in the drum prior to adjusting release clearance.

I've been working with the old-style bodies and Dana70HD's and the big pre\early '90's corporate's, (excuse) and the Dodge Dana80HD systems, and it all tends to blend together after a bit.

(No need to bring up the "s" word, right?)

So, considering that you're posting correct data on the trucks you guys drive, Jeff, then I must needs defer to your input.

'Cept for the method I use for adjusting shoe clearance, of course. :cool:

[Robyn]

Just my 2 cents worth.
I have always adjusted brakes up tight and then back of untill just free to turn without drag. I have never had one issue ever. I learned the brakes thing from all my big truck stuff (down tight then back off 1 /4 turn)
That is on air brakes with S cam type.
Always do my small stuff tight then back just to free.
Hmmmm never had one grab unless its wet or lost a seal and got oily.

Robyn

[noelb]

Thanks, I take it the park brake on you Burb or trucks is activated by the foot - Right? Mine is by hand. I am taking all of this info in - slowly though! Make take a few days... For me and my personality it is imperative to know how things work before I touch them. I have to know why I am following a particular procedure.

I have adjusted the brakes up and then back 33 notches. 1 turn of the adjuster (Marked a white mark as per Jeff's instructions)+ a further 7 notches approx. Brakes seem good but then the weather isn't wet!

Thanks GMCTD, Jeff, Robyn and others.

[Mainous]

I quickly assembled a webpage after sending Noel some pics from the manual. There is an ongoing misunderstanding of these brakes and this same type of thread is on the forum every so often with pretty much the same discussion. Maybe this will help.

http://home.cinci.rr.com/mainous/Duo-ServoBrakes.htm

Thanks

[gmctd]

Jeff, your patient insistence in relating the facts had me thinking, puzzling - then, there it was, like the Bat Signal in the sky, a picture in my mind of the brakes on the Malibu, S-10, and other lite-duty GM diff's.

The bottom of both shoes fit into a grooved V-block - they are permanently centered in the drum - leading-trailing tech.

I designed many different-type brackets to convert the Ford 9" diffs to my project-cars, incl adapting the Lincoln rear disk brakes and the Toronado\Seville rear disk setups, to get away from that type brakes.

Again, we need not bring up the 's----e' word and the old-age syndrome, right?

We're on the same page, and the right Page, I think.

[Mainous]

gmctd,

I knew we would get there eventually. Thanks for your input.

[joed]

One thing I've noticed is that the self adjusters work a lot better if you can back up with some speed (10 mph) and hit the brakes hard - doing this about 3 times in a row. I live near a quiet street and periodically do this - the rears seem to stay tight. Normal back-up speed doesn't seem to be as effective.

My dad's 95 truck's rear brakes were grabbing and after doing the back-up with speed manuever, they seem to work a lot better.

Joe.