This Thing Will Not Stop

[GOT DSL]

I have read many posting on the Diesel Page discussing the lack of stopping power inherent to trucks with 6.5l diesel engines. My 97 Suburban is taking way to long to stop and I was wondering before I start taking brakes a part if anyone had found a good fix?

[rustypig]

Hmmnnn..well, yes and no.
From everything I've read, you can do just about everything...ie..change proportioning valves, go to different brake pad materials, cross drilled rotors, do some work on hydra-boost system, and in the end...the braking action is better, but still leaves something to be desired on a C/K version GM.

I've planned (for over two years) to disect my hydra-boost and look for ways to improve braking. Always something more important to do it seems.

Kennedy does offer something to help out here, I just haven't tried it myself.

I noticed that going to carbon fiber break pads helped some, but only after they are initially "warmed up".

If you find the miracle pill.....let me know!

[Mainous]

Is your truck a 1500 or 2500? I'm not totally positive about the 1500 but all of the 2500's have duo servo rear brakes. Most service departments, including dealerships, don't work on these enough to know how to properly adjust them and therefore they don't work right. They should be adjusted until they start to drag and then backed off 33 clicks. If they aren't adjusted properly, there isn't enough room for the shoes to rotate and only one shoe bites. I have never had any complaints about stopping power on these brake systems if everything is working properly.

[moondoggie]

Good Day!

Mainous: You said, "If they aren't adjusted properly, there isn't enough room for the shoes to rotate and only one shoe bites." Could you please explain? To the unenlightened (me), it seems like backing off the adjusters as you describe would provide extra room between the shoes & the drum, which all goes away when you press the brakes. The wheel cylnder will push both shoes out until they hit the drum - the extra room is all gone.

I'm pretty sure I completely missed something that's really simple here. Have a good laugh, then please explain what I'm missing.

Blessings!

Brian Johnson, #6042

'82 6.2 1500 4X4 1/2T pickup, 4spd man w/ OD, bone stock, 335K+, "In Rust We Trust"
'89 6.2 4X4 1/2T pickup, bone stock, 145K+
'95 6.5TD 2500 4X4 heavy 3/4T pickup, Gear Vendors Aux. OD, 180K +
'95 6.5TD 1500 4X4 3/4T Suburban, Kennedy exhaust, 200K +

[cruzer]

What's duo servo?
If you back off the adjustment you will usually
loose pedal height.

[HowieE]

Mainous I am with the guys above in that I have never been able to get rear brakes on my 97 2500 Sub. I gave had it into what I consider one of the better GM shops, a local shop that I have done business with for 30 years and played with them myself to no avail.

Yes the 97 GM manual says adjust till drag and back off 33 clicks. I have little or no brake peddal now and would assume if I back off that far I would be on the other side of the firewall before stopping.

I have changed the proportional valve and installed Praise pads and shoes. Just before my last trip I adjusted the rear brakes as I have for years. Tight and back off 3 clicks. After 9500 miles when I got home I adjusted each side in 20 clicks before they were tight and then out 3 clicks. Had no rear barkes before or after of note.

Hope you can shed some light on this subject.

[Mainous]

First a quick description of standard drum vs. duo servo brakes. Standard drum brake shoes pivot off of a center post at the bottom of the backing plate. The wheel cylinder expands and both shoes spread out engaging the drum. Duo servo brakes do not have this post at the bottom of the backing plate. When the wheel cylinder spreads,the rotation of the drum actually wedges the shoes and makes the whole assembly rotate with the drum. The "wedge" action ceates more brake force as more force is applied. Backing the adjuster off gives room for both shoes to fully engage the drum. If they aren't backed off, only a portion at the top of each shoe engages the drum. These brakes also have a leading and trailing shoe. The metal part of each is the same, however, the lining is different from leading to trailing. It is very easy to totally screw up the rear brakes on one of these trucks if you don't pay attention to what's happening with this type of system but once you understand the way it works it mkes more sense. Hope this helps.

[bigredhummer]

I am pretty sure it is NOT the hydroboost. I have the same hydroboost on my 6.2L humvee and the brakes are extremely powerful, locking all 4 up is easy, slowing down rapdily requires only moderate pressure, and easy stops, just resting your foot on the pedal. I would think that the braking problems you are describing do not come from the hydroboost.

[rjschoolcraft]

As a matter of note...

My Suburban has always stopped quite well. I have had some problems with front rotors, but it always has stopped well. That's why I am really surprised to hear all of this talk of poor stopping performance on the DP.

Mainous has described the duo servo type of brakes quite well. The shoe with the longer friction material goes to the rear.

I don't usually go the full 33 notches back. I back them off until I have no drag, then a little more. They seem to work very well.

[matt-max]

at their best, our brake systems are lacking.

drive a new generation gm truck with 4 wheel discs and you'll see, if you haven't. on the other hand, don't drive one or you'll want to trade.

their is a 4-wheel disc conversion using older gm parts that i have notes on somewhere. can't remember if i got the info from this page or not.

seems there would be a way to use the new style parts as well if the rear axle parts are similar.

matt

[rjschoolcraft]

Just curious...

How much better can it get, if a panic stab on the pedal activates the ABS on all four wheels?

[tom.mcinerney]

I had my brake master cylinder lined with a SS sleeve. The shop had to get a new boring bar to reach into the long cylinder.{My '95 also has the duo servo}. The longer cyl may be a design to accomodate the extra play of these shoes.
When drum brakes are involved, new drums & shoes yield a far firmer pedal and stop much better because the cylindrical shapes comform and much more surface contacts . It seems like the shoes warp under pressure when they're a bad fit in worn drums.
Four wheel discs was an option in '95.

[GOT DSL]

The discussion from the memebers has been great and informative and Mainous did a great job of explaining duo-servo brakes. Thank you!

While attempting to understand why my rear brakes are not stopping hopefully I can also help others with similar issue. Maybe prompt more discussion too!

I just happen to be an engineer and have access to the guys that design braking systems for a certain truck. So I asked questions.

The suggested clearance from brake shoe to drum is .030" that would be more like 3.3 clicks off the adjuster not 33 clicks. I know, my manual says 33 clicks too! My plan is to adjust the brake shoes out until there is drag at the rear wheel and then back off the adjuster until the drag goes away.

My weekend job is to get the rear wheels off the ground. With engine running at 1000 ~ 1500 rpm, trans in drive, see if the brakes will stop the rear wheels from spinning.
Next step? Monday

[cruzer]

I've never seen american drum brakes w/a post
on the bottom of the backing plate.(p/u truck
or car) the post is usually at the top just above the wheel cyl. On the bottom is the brake self- adjuster. I've always adjusted brakes with adjuster to a slight drag,and never backed them off. After a few miles the drag will go away and ensures they are adjusted evenly. It also helps the adjusters keep the pedal up. It's always worked great for me and the brakes work fine. The self adjuster works when you apply the brakes going backwards. I would think if you back the adjuster off too much the adjuster wouldn't be able to recover to keep proper adjustment. I've never had to adjust my brakes after a brake job. It maybe wrong, but it works for me. That's my 2cts.

Checkout pirate4x4.com You will find how to convert disc brakes for a ff 14bolt using GM parts.

[billg]

There still must be some type of brake shoe anchor in the duo-servo system. I agree with most brake shoe anchor posts are at the top, but the point was made regardless. I personally check the rear adjustment between oil changes and adjust to the slight drag method. This made a big difference in the rear wheels sharing in the stopping of my truck (especially with a load). I can see that maybe I

[moondoggie]

Good Day!

I did rear brakes on my future son-in-law's '95 (?) Taurus, & I was shocked to see a post on the bottom, where all my GM's have the adjuster (the adjuster was on top - hard to explain, a pic would be better). The last time I saw this type of brakes was a LONG time ago, when I was working on air-cooled VW's.

My foggy understanding is that those old VW's & this Taurus only self-actuate the leading shoe - the trailing shoe only plays a minor part. I thought that, before disc front brakes, they built most cars with the no-post type drum brakes on the front axle (for maximum braking power when moving forward), & the bottom-post type on the rears to assure adequate braking when backing up. This bottom-post design was abandoned (by everyone but Ford, I guess) with the advent of disc brakes, as they work as good going forward or backward.

Clarification, please?

Blessings!

Brian Johnson, #5044

[billg]

Checking to see if my signature made it.
bg

[HowieE]

Well I just went out and removed the drums to havwe a look see. There is a centering/anchor pin at the top with the cylinder just below that. This would let the cylinder expand both shoes against adjuster and once in contact with the drum rotate slightly until coming in contact with the anchor pin.
Now the question still remains, is it 3 clicks or 33 clicks back after drag? I just tried 33 and the peddal went right to the floor.

[cruzer]

Old VW's didn't have self adjusting brakes.
On GM and old fords the pin was/is on the top.
The back shoe is the primary shoe(the big shoe).
The smaller front shoe is the secondary shoe.
Mainous has it right on the operation. The brakes
are applied pushing the pistons out that, energizes the shoes and the shoes rotate slightly. The back shoe is doing more work.
You have to remember the pistons are pushing against each other they do not have separate cylinders. The rear shoe at the top will eventually push against the pin on top and everything rotates accordingly. The shoes should come in contact with the drum fairly evenly. Maybe a little more in certain spots when the brakes are intially engaged. That's the way I see it.

Brake sdjustment=pedal height. When you back them off you will loose pedal height. I personally
don't back them off. I adjust to slight even drag
rotating the tire one or more complete turns, so I know the shoes are in contact w/the drum all the way around. Then adjust the other side w/ the same amount of drag. This way the vehicle wont pull one way other because of misadjustment.The slight drag will go away in a few miles and shouldn't be felt when in gear(it will still roll). We did it this way forever in the shop.We never had a problem, and never had to readjust brakes unless the adjuster froze up.Be sure to clean and grease the adjuster so it works properly during installation. This works 4 me.

.

[ 07-25-2003, 03:34 PM: Message edited by: cruzer ]

[Mainous]

Guys,

It's definintely 33 clicks. I have adjusted them both ways and they will grab and go into antilock if they aren't adjusted right. They will also warp the drum in a short amount of time because all of the heat is concentrated in one spot instead of the entire circumference of the drum.
Follow your manuals. It's hard to get the 33 clicks dead on because you have to push the stop off the adjusting star so it doesn't actually click. But you will get a feel for the amount of travel one turn of the brake spoon will get you pretty quickly. If you have had the shoes off make sure that the shoe with the most material at the top is toward the rear of the truck.