View Full Version : This Thing Will Not Stop
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
07-23-2003, 20:40
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!
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
07-24-2003, 08:26
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 +
What's duo servo?
If you back off the adjustment you will usually
loose pedal height.
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.
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
07-24-2003, 15:34
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
07-24-2003, 15:41
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
07-24-2003, 16:01
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
07-24-2003, 20:53
Just curious...
How much better can it get, if a panic stab on the pedal activates the ABS on all four wheels?
tom.mcinerney
07-24-2003, 22:33
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.
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
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.
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
07-25-2003, 10:55
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
Checking to see if my signature made it.
bg
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.
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 ]
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.
I just had a bit of an epiphony. One of you guys that has your drums off needs to count the number of points on the adjusting star. This will equate to a given number of clicks. If we can equate 33 clicks to a given amount of rotation of the star maybe this will be easier for all of us.
Adjust them til they drag. Mark the adjuster with some nail polish (or equivalent) and turn the appropriate amount. Make sense?
rjschoolcraft
07-25-2003, 21:51
I had mine off two days ago to replace wheel seals again (I went back with CR seals and Speedi Sleeves this time) and brake shoes again (saturated left rear). I didn't think to count the teeth on the adjuster, but I'm gonna bet that 33 clicks is about one turn. I don't intend to pull the drums to count, but would appreciate if someone is into the brakes on a 10.5 inch diff...please report. Otherwise, next time I have them apart, I will definitely count.
Had the back end of the suburban jacked off the ground and the rear wheels spinning. When I appied the brakes the left rear wheel did not stop, the right wheel did stop. Removed the inspection plugs and found the left rear brake shoes were coated in oil. Looks like there might be a bad seal or a wheel cylinder leak.
There was not much clearance between the drum and brake shoes and they were not rubbing. I will check the number of clicks on the adjuster before taking the brakes apart.
tom.mcinerney
07-28-2003, 21:17
Bill G-I've also puzzled over the issue of drum brakes on big trucks; no answer, but offer the following: On race cars discs were adopted because were nearly immune to fading, which drums are prone to. They use air to actuate big-rig drum brakes, it's crude, but effective.
Cruzer-At an annual state safety inspection i was once cautioned not to adjust with drag, the fluid can boil.
Got DSL-Don't let any of your brake engineers near me when i'm working brakes! Please beg them to make sturdy bleeder screws with thick walls(thin bleed drilling) preferably from corrosion resistant high alloy(M 10.9/12.8?). I hate busting those suckers....
-i bought my truck with both large shoes on left rear, both small shoes on right rear!
Lets keep this issue open until someone comes up with a reasonable answer that can be defended with some level of science.
I had my truck into a GM shop yesterday for a rear seal and thus a brake adjustment as part of the reasembly. I ask the mechanic how he had adjusted the brake given our open discussiion. He adjusted till drag and did not back off at all. Claimed that was how they always did it. I got less than 50 miles down the road before the left rear started to lockup. Limped home and backed off on that side about 3 clicks. Will see what happens today as far as stopping power, peddle height, and lockup.
Yes with that adjustment I did have a reasonable peddle when I let the shop.
Another point of interest with respect to the rear brakes. If I attempt to stop the truck with just the parking brake while at a slow roll there is almost NO stopping power at all. I would have assumed I could lock the rear with full pressure on the parking brakesat a slow roll.
I mentioned a year or so ago that when I lost a brake line that the proportioning valve was not worth a dam in that I lost a brake line to the front and with useless rear brakes you have but one choice. With the steering wheel in front of me I just couldn't bend over to get there.
I do not think that adjusting brake pads out until there is drag between the shoe and drum and not backing off the adjustment is a good practice.
This method of adjustment does not allow for thermol expansion when parts get hot, seems like a good way to warp a drum.
Using the emergency brake does not expand the brakes the same way as the wheel cyldr. Also
a slow roll may not be enuff to energize the
shoes. Also hydrolics are going to apply more pressure to the shoes.
Here's another one you'll not like. A friend of mine has a 2000 k2500. He didn't like the way
his anti-lock brakes worked, so he disabled the
ABS by pulling the fuse. His brakes work better now.
HowieE's experience is one I've seen repeated on several occasions through friends with the same trucks. The GM service manuals call for backing them off 33 clicks. Again, I know that it can be hard to estimate 33 clicks because of how they have to be backed off. But if done properly the
brake pedal is excellent and they stop great.
GOT DSL are you going to do your own brake work?
If so, count the points on the adjusting star and let us know how many there are. This will bring perspective to this discussion. I think ronniejoe's estimate of one turn will be pretty close.
One other side note...another thing I have done to my truck to improve pedal feel is to install braided stainless brake hoses all around (3 total). This gives a much more solid pedal when stabbing them and takes the vibration out when the anti-locks kick in. "Russel Hoses" makes these and a set is around $70 from Summit or Jeg's.
Mainous - Yes, I am going to do my own brakes. Counting the teeth on the adjusting wheel is not a problem. I will also determine how many clicks the adjuster is set at before removing the drums.
It should be interesting cause last time I had the drums off I backed the adjuster off the 33 clicks per the service manual.I suspect the auto adjustment has change the adjuster position to less than 33 clicks.
I've got a friend with a mid to late 90's truck, brakes always gave him problems, almost to the point of no brakes at all. He took out the abs fuse and the breaks work great now.
Guys,
I created a PDF file with this particular section of the GM repair manual. I can't imagine the General would get too upset at it getting used in this forum since we are trying to clarify how to do proper maintenance on one of their products.
Some of the terminology I used in my original description with regard to leading/trailing shoes was wrong since that is the terminology used for the "standard" brakes. The proper terminology for the duo-servo brakes would be primary/secondary shoes. Other than that I was pretty close.
This doc describes both systems. Remember, we are dealing with the duo-servo type. Here is the link:
http://home.cinci.rr.com/mainous/BrakeDoc.pdf
rjschoolcraft
07-30-2003, 23:54
HowieE:
Your park brake should lock the rear wheels when rolling slowly. I tested this yesterday after reading more of these posts. I rolled the Suburban at about 5 mph and applied the parking brake...not a panic stab, just a moderate depression of the pedal. My rear brakes locked up and slipped the wheels slightly on pavement before the truck stopped.
If your parking brake isn't working, it could be the brake adjustment that has been heavily discussed here, or it could be that your parking brake cable needs adjusting. I just went through that since I had to replace the cable going to the right rear wheel. It seems that it snuggled to closely to the exhaust pipe and melted down. The cable stuck and wouldn't allow the brake to release. The adjuster on my Suburban is below the left rear door where the individual wheel cables come together.
I will say again...I don't buy this "the brakes on these trucks are no good" stuff. I think it is all about proper adjustment and proper pads/shoes. I also think that these master cylinders have more pedal travel than most people are used to and have a spongey feel. My Suburban weighs about 6500 lbs. by itself and it stops very well (once you're used to the feel of the master cylinder). Some of you can say I'm ignorant (and you are probably right), but I have driven lots of other vehicles and would rate my Suburban's stopping power very highly. BTW, Kennedy mentioned an upgraded master cylinder when I drove his truck. It had a much better pedal feel. I would like to know what he's using.
I have a 1967 C10 pickup that originally had manual drum brakes on all four wheels. These drums didn't even have any cooling fins. Now that took a stout leg and plenty of room to stop, especially when towing a trailer! :eek: Now it has the power front disc/finned rear drum set-up from a 1971 C10. What a difference.
Again, panic stabs of the Suburban brake pedal will activate the ABS on all four wheels. This says that the brakes can apply significantly more stopping torque than the tires can transfer to the pavement. What more can we ask for?
moondoggie
07-31-2003, 11:15
Good Day!
Mainous: Thanks for the pdf. Even though I didn
Some new information for the equation. The star wheel has 27 clicks per revolution, which measures to about 1/16 in separation at the bottom of the shoes.
As I mentioned above I had a GM dealer replace the rear seal last Monday and they adjusted the brakes to drag with no back off. One brake locked up on the way home so I opened it up today to find a leaking seal. Man I love to pay for this kind of service.
When I put it back together I did adjust as per the manual and back off 33 clicks. It appears there may be 2 reasons for this excessive number of clicks. The Lawyers have gotten in to act in that all brake materials do not expand at the same rate when hot and this looks like the safe number to cover all materials. A second and hopefully a more intelligent reason is the self-adjuster. With this amount of clearance the self-adjuster lever can be moved enough to engage the next star wheel notch. I hope it
The resriction you mention could be the residule valve, which is a one way valve. It restricts the fluid going back into the master clydr IIRC. It keeps slight pressure in the line. I know if you were going to modify to disc brakes that valve has to be removed because it would keep slight pressure on the pads and the rotor and cause to pads to wear prematurely. The valve should be between the output line and the piston output in the master clyinder for the drum brakes.
I think you'll find you will have to pump the pedal or loose pedal height if removed.
[ 08-01-2003, 08:55 AM: Message edited by: cruzer ]
BillG and tom man 95 , Disc brakes for large trucks will soon be common place. New brakeing regulations are, shorter stopping distances, going to force better brakes. Discs are used often now on medium duty hydraulic trucks. Air brakes are the preferred design on larger trucks and air discs have had problems but these problems are being solved. Discs are comeing for many of the same reasons they are on cars and light duty trucks. My K20 stops great, Raybestos or ACDelco friction material only. bob
Pulled the left rear brake drum off my suburban over the weekend and found that the brake shoes are coated in oil. Looks like the axle seal is leaking.
Have a question: Since the axle seal needs to be replace should the wheel bearings be replaced? The suburban has about a 180,000 on the odometer and the bearing have never been replaced.
rjschoolcraft
08-04-2003, 09:30
Probably not...
The most common failures in bearings these days are due to contamination or manufacturing defects that result in "infant mortality". Statistics show that bearings that have been in service, pulled and inspected then returned to service are much less likely to fail than new bearings.
Look them over well and if you don't see any indications of distress (pitting, discoloration from heat, or skidding marks) return them to service.
Inspect the seal journal for damage as well. You might want to consider using CR seals and Speedi-Sleeves.
The axle seal has been replaced, new brake shoes installed per the process in my service manual. After replacing brake shoes to set clearance between the shoes and drum it says to adjust while spinning the wheel until you can just turn the wheel by hand. Back off thumb wheel 15 clicks, wheel should spin freely. Continue backing off thumb wheel a total of 33 clicks. There was about 1/8 of an inch clearance between the brake drum and
brake shoes after completing this process. So, I adjusted out 15 clicks and the brakes work better not great. I am going to let them adjust themselves and check the final clearance.
What is next?
You got me confused. How many total clicks did you back them off?
britannic
08-12-2003, 17:04
Originally posted by GOT DSL:
Pulled the left rear brake drum off my suburban over the weekend and found that the brake shoes are coated in oil. Looks like the axle seal is leaking.
Have a question: Since the axle seal needs to be replace should the wheel bearings be replaced? The suburban has about a 180,000 on the odometer and the bearing have never been replaced. Was there gear oil on the drum and linings and if so, what did you clean it off with?
Mainous,
At the end of the adjustment process per the service manual the total number of clicks that the thumb wheel had been backed off was 33. Then I turned the thumb wheel back 15 clicks. So, 33 – 15 = 18.
GOT DSL,
Remember what was said earlier. If you don't back the adjustment off 33 clicks you won't get the full surface of the shoes engaging the drum. Yes, it does seem like this is a lot. But if the shoes can't rotate you are not recognizing the benefits of the duo-servo design (wedge effect).
The axle seals have been replaced, new brake shoes installed, the clearance between the brake shoes and drum set and this thing still doesn’t stop. Talked to the engineers again they say that I should be able to lock up the brakes or at least get the ABS to cycle, my truck will not do either one. The brakes are working at all four corners they just don’t seem to up to snuff. The brake pedal also feels soft. Think my next step will be to see if the Hydroboost is functioning correctly.
Just wanted to chime in here. My "96 Suburban seems to have the same problem as GOT DSL. The truck will stop, but in a panic stop, the ABS does not seem to engage. The pedal is on the floor almost immediately with moderate stopping power.
This is my first hydro-boost with ABS. Should the pedal pulse? Also, it seems you would hear some sort of chatter when the ABS is doing its thing. I have none of these signs.
In fact, the only thing that happens is that I dump transmission fluid on the exhaust and street in a hard stop...That is for another discussion.
Think my brake problem is not in the adjustment of the rear brake shoes but a matter of not getting sufficient pressure to actuate the brakes. In my service manual there is a section that describes two procedures to test the brake booster. These test procedures have led me to believe that my accumulator is bad. The accumulator is the brass colored cylinder on the inboard side of the brake booster. One section of the test says; with the engine off press the brake pedal several times to empty the accumulator. If the accumulator has lost its gas charge it can be rotated or will wobble. If this condition exists replace accumulator. There is another test that tests the valves inside the booster, which my truck failed also. Think that I am going to consider replacing the booster.
This thing stops now!
Replaced the booster and master cylinder and what a difference it has made. I can almost get the brakes to lock up on a dry road. Can’t wait until it rains to see if the ABS is working.
rjschoolcraft
09-29-2003, 09:15
Congratulations! It is good to hear of a happy ending! smile.gif
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