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Bus Discussion => Bus Topics ( click here for quick start! ) => Topic started by: solardude on June 09, 2018, 09:29:31 PM

Title: Break slack
Post by: solardude on June 09, 2018, 09:29:31 PM
I have finished all the rear radius rod bushings, and all other associated repairs, I am now preparing to remount the wheels. However in the process of removing the drums, I backed off all the breaks to make drum removal easy. I have been told the proper adjustment of the breaks is to tighten slack adjuster till breaks are locked up, then back off 180 degrees. So I counted the "clicks" 180 degrees was 38 clicks.
So I plan to do that to all 4 of the rear wheels in hopes of even breaking.

Does that sound correct?
What is your method for slack adjustment?

SD
Title: Re: Break slack
Post by: eagle19952 on June 09, 2018, 10:12:11 PM
Are you talking auto slacks...?
Title: Re: Break slack
Post by: solardude on June 09, 2018, 10:29:53 PM
Not auto slacks, manual.
When in doubt: read the manual. The manual says to tighten and back off two clicks in front and three in back. Answered my own question.

SD
Title: Re: Break slack
Post by: richard5933 on June 10, 2018, 04:04:41 AM
You've got to be careful to follow the proper front-to-back recommendations. When the shop working on our 4106 first did the brake adjustment, the tech followed what he thought was the right way and set them all the same. Problem was that the bus designed for something else. Front brakes were being asked to carry too much of the braking load and quickly overheated, causing a near-disaster on the way home. I turned around, had them crack the manual (and call Luke for confirmation) and then readjust. No problem after that. This is not something that should be done based on what seems right, but rather strictly by the book.
Title: Re: Break slack
Post by: Utahclaimjumper on June 10, 2018, 05:47:53 AM
 If you keep "breaking" your slacks, its going to get expensive. >>>Dan
Title: Re: Break slack
Post by: chessie4905 on June 10, 2018, 06:21:32 AM
Before reattaching the wheels, check for bearing looseness at rear hubs. Loose bearings can cause drum drag when putting coach back down on the wheels.
Title: Re: Break slack
Post by: brmax on June 10, 2018, 07:01:53 AM
Make sure all air tanks are full when taking on this job. This is very important on spring brakes and a good pattern with the others as well.
I was taught the wheel needs to be off the ground when checking and adjusting. I still like to follow this method for a rolling sound check for scraping and or misalignment of shoes and somtimes winter weather shoe issues.
The cdl brake visual check should be a step in this adjustment and followed. I found a wood handle wedged in place for brake applications work well as theres not always another helper around. :)


Good day

Floyd
Title: Re: Break slack
Post by: buswarrior on June 10, 2018, 07:03:25 AM
The entire sorry history of slack adjusters must be observed and respected, when we start tossing "advice" about brake adjustment.

You may have already damaged certain slack adjusters, by your methods.

A busnut MUST identify the particular brand and model of slack adjuster that some previous owner may have installed, compare that to what the coach manufacturer installed, and RESEARCH the correct methods for their install, ongoing maintenance and how to both back them off, and re-adjust.

No kidding, there have been auto slacks that using "traditional methods" breaks their internals.

The consequences of doing this wrong are setting the coach on fire, via a dragging brake setting the tires on fire... or, stepping on the brake pedal and the coach not stopping as you want it to...

One size does NOT fit all, as we approach the mid point of 2018, and busnuts are deep into the variety of auto slacks that came out back 25 plus years ago, some for only a brief and convoluted time.

PLEASE be SMART, then you will be SAFE!!!

Happy coaching!
Buswarrior
Title: Re: Break slack
Post by: windtrader on June 10, 2018, 10:15:06 AM
great advice, buswarrior! Some may take comfort having a manual and following the instructions which were written for OEM equipment while in fact what is installed after 30-50+ years can be something different.
Title: Re: Break slack
Post by: eagle19952 on June 10, 2018, 10:17:15 AM
No kidding, there have been auto slacks that using "traditional methods" breaks their internals.

personally i think these should be illegal.

getting under your bus and snooping around should b a regular deal'

Owning a bus and not able or willing to adjust your own brakes is  fools game.

Back in my gravel hauling field mechanic days...I handed drivers 9/16" wrenches and a tutoring moment.

or a pink slip and a check.
Title: Re: Break slack
Post by: TomC on June 10, 2018, 11:21:12 AM
Always adjusted the brakes down to stop then back off 1/3 turn. I've never heard that adjusting brakes differently from front to back made a difference-after all, these are air brakes. Compared to hydraulic brakes that do move in direct proportion to how much pressure is applied. Good Luck, TomC
Title: Re: Break slack
Post by: richard5933 on June 10, 2018, 11:29:43 AM
Always adjusted the brakes down to stop then back off 1/3 turn. I've never heard that adjusting brakes differently from front to back made a difference-after all, these are air brakes. Compared to hydraulic brakes that do move in direct proportion to how much pressure is applied. Good Luck, TomC

Makes a huge difference to some coaches - the 4106 is has different sized drums front and rear, and obviously the larger ones are designed to do more of the braking. Setting them the same front to rear is what caused the smaller drums to heat up faster than they should and fade quickly. The way it was explained to me is that the larger drums should engage first and carry more of the weight when stopping. I'm no brake expert, but I followed Luke's advice on the 4106 and now on the 4108 and stopping is much better.
Title: Re: Break slack
Post by: luvrbus on June 10, 2018, 12:14:52 PM
Always adjusted the brakes down to stop then back off 1/3 turn. I've never heard that adjusting brakes differently from front to back made a difference-after all, these are air brakes. Compared to hydraulic brakes that do move in direct proportion to how much pressure is applied. Good Luck, TomC


Must be a GM thing TomC,how in the hell would a automatic slack adjuster adjust to that ? maybe GM's don't have propositioning valve
Title: Re: Break slack
Post by: eagle19952 on June 10, 2018, 12:26:04 PM
on ice..front brakes can/will hurt you.
many (in the Arctic off road) are disabled.
Title: Re: Break slack
Post by: luvrbus on June 10, 2018, 12:30:17 PM
on ice..front brakes can/will hurt you.
many (in the Arctic off road) are disabled.


For years tandem trucks never had brakes on the front axle,some older buses had a valve to disable the front brakes or reduce the air pressure
Title: Re: Break slack
Post by: eagle19952 on June 10, 2018, 03:49:24 PM


For years tandem trucks never had brakes on the front axle,some older buses had a valve to disable the front brakes or reduce the air pressure
yup. :)
Title: Re: Break slack
Post by: DoubleEagle on June 10, 2018, 07:39:56 PM
And there is the issue of adjusting the tags or bogies to work evenly with the other positions to avoid wheel lockup because they are not loaded enough compared to the drives and front.
Title: Re: Break slack
Post by: eagle19952 on June 10, 2018, 10:09:02 PM
And there is the issue of adjusting the tags or bogies to work evenly with the other positions to avoid wheel lockup because they are not loaded enough compared to the drives and front.

All for not  if the treadle valve is gummed up...or the ....
Title: Re: Break slack
Post by: buswarrior on June 11, 2018, 06:57:08 AM
There is some wrong thinking in here that could get busnuts into trouble.

The air pressure applied to the chamber determines the strength of the squeeze on the brake parts.

The slack adjuster is just a lever that does two things, it strengthens the chamber's output to twist the S cam harder, and allows a certain amount of brake wear, and provides the adjustment point, before the brake chamber stroke has been used up and can't reach to squeeze the brake parts anymore.

The adjustment of the lever being talked about on this thread doesn't change the strength of the squeeze. (Of course, unless you loosen off so far that the brake parts can't squeeze, and then the coach won't stop properly. You need to quite naive to do this on purpose...)

Now, again, a busnut must be aware that previous owners will have slapped any old parts in there, to go make more money, or save some money, with neither a care, nor the knowledge that they have compromised the design.

The brake chamber size, chamber push rods cut to correct length, the proper relay valve, working correctly, the proper length slack adjusters, the treadle valve under your foot, proper air line fittings and hoses to supply the correct air VOLUME to the brake components...

Something as simple as adding an elbow fitting where the manufacturer put a loop changes the timing of air arrival. Changing air line diameter has dramatic effects on the arrival time of the volume of air to fill the chambers.

Buses are NOT trucks. Buses have a more consistent, and heavy, front axle loading than trucks of days gone by, and no facility to grossly load the rear. Front axle skidding in wet conditions is not the same issue for a coach.

Adjust your wheels end components according to modern published procedures, and confirm the rest is the way it should be, and enjoy a coach that slows and stops as you will want it to.

happy coaching!
buswarrior

Title: Re: Break slack
Post by: luvrbus on June 11, 2018, 07:13:39 AM
What is being said is you can not get the same braking pressure from a 16,20 or 24 service brake chamber as you can from a 30 chamber on a rear axle this GM deal makes no sense what so ever.I have replaced auto slack adjusters on GM's before and it did not know if it was on the front axle or rear axle.The size of the chamber and shoes dictates the braking pressure on any air brake system or Bendix missed the boat     
Title: Re: Break slack
Post by: chessie4905 on June 11, 2018, 07:18:25 AM
Richard, I'll bet you misunderstood what Luke told you. Buswarrior has it right. The brakes aren't sequential. They should all apply at same time and evenly. If your front brakes overheated, something wasn't right. Just a guess as to what the problem actually was after an issue in the past..
Title: Re: Break slack
Post by: richard5933 on June 11, 2018, 10:25:46 AM
I'll have to double check with Luke, but I'm pretty certain that he said to tighten the adjuster and then back off a specified amount - different for front and back. My notes with details are not accessible at the moment, but I know that the shop did them all even front to back the first time on our 4106 and it caused a problem. When they did them as suggested with a difference things worked better.
Title: Re: Break slack
Post by: eagle19952 on June 11, 2018, 10:34:10 AM
Just because the splines match on the slack and the S cam doesn't mean you have the OEM spec'd slack.

Title: Re: Break slack
Post by: brmax on June 11, 2018, 11:30:32 AM
Thats a good point Don many have up to three holes for chamber rod/pin mount 👍  Also assuming one may well have left or right mount slack adjusters.

Richard you can adjust them tight and back them off so not to scrape, this amount to however much you want. As long as when done adjusting and a brake application is made as in procedural guidelines the rod movement measures within DOT specifications.

I like it when manually checking rod movement with a pry tool a measurment of 1/2” - 3/4”, again this in my opinion is best done with that particular wheel off the ground and able to turn by hand.

Good day there

Floyd


ps: the little plastic c clip washers for the chamber rod measure is the best thing ever, plus its a seal of sorts
Title: Re: Break slack
Post by: Lee Bradley on June 11, 2018, 11:49:52 AM
What is being said is you can not get the same braking pressure from a 16,20 or 24 service brake chamber as you can from a 30 chamber on a rear axle this GM deal makes no sense what so ever.I have replaced auto slack adjusters on GM's before and it did not know if it was on the front axle or rear axle.The size of the chamber and shoes dictates the braking pressure on any air brake system or Bendix missed the boat     

My Cityliner has the same brakes on the front and tap axle; difference is the tag has smaller chamber to reduce the braking also has smaller air springs because it carries less load.
Title: Re: Break slack
Post by: bevans6 on June 11, 2018, 12:43:13 PM
Fun with numbers...  I did this when I took my air brake course so I could more fully understand what was happening.  There are five things that impact the amount of force being applied to the shoes inside the drums.  First is air pressure - the foot valve can apply a varying amount of air pressure to the brakes.  Assume for the sake of argument you are applying 100 psi (a full brake application).  Second is chamber size.  Most rear drive axles will have a 30 square inch chamber, so for your full brake application (such as when testing push rod movement for a DOT inspection) you will have 3,000 lbs of force on the push-rod.  This is one reason why pulling the slack back by hand or with a pry bar isn't a good test.  3,000 lbs finds problems that 25 or 50 lbs does not.  Third is length of the push rod.  It is designed so that there is a 90 degree angle between the slack adjuster and the push rod when the slack is correctly adjusted and there is a full brake application.  Maximum force is transmitted at 90 degrees.  If the push-rod is too long or too short (usually a mistake when installing a new brake chamber),  you will not get maximum designed braking force applied to the shoes.  Fourth is length of the slack adjuster lever arm.  Some slack adjusters have two or three holes in the arm at different distances from the splined hole.  You need to have the push rod clevis in the right hole, and same on both sides, or you will get uneven braking side to side or front to rear.  The distance acts as a multiplier for the force - assume a 6" radius for the slack adjuster hole and a 1" radius for the S-cam application point (in the ballpark of right) and your 3,000 lbs of push-rod force acting through a 6:1 leverage ratio becomes 18,000 lbs of separating push at the brake shoes.  The fifth thing is the S-cam itself.  It will apply force at a varying radius as it and the brake shoes, drum, bushings wear, changing that 6:1 leverage ratio.  If the brake shoes and all else wear too much, the S cam can fall off the brake shoe pin, you get what is called "over-cam" and you lose brakes on that wheel until you take it all apart and fix whatever broke or wore out to let it over-cam.

Neat to understand the magnitude of the forces released from a 25 lb push on the brake pedal.
Title: Re: Break slack
Post by: buswarrior on June 11, 2018, 01:50:55 PM
And, the true devil... the relay valve down the back...

Is it putting the proportional pressure and volume to the rear brakes, that the treadle valve is calling for?

The Europeans are way ahead of us, ports installed on the vehicle to test the application pressure of the relay valve.

When this was being checked on these shores back 15 plus years ago, you really don't want to know the variability of "out of the box" new valves...

And since there's been no further regulation of these matters since then...

Don't expect things will be much changed?

After a big accident, the Federales will be testing the relay valves as part of the forensics...

Maybe I need to poke my nose into this again....

happy coaching!
buswarrior
Title: Re: Break slack
Post by: Friday1 on June 12, 2018, 06:13:52 AM
Thats a good point Don many have up to three holes for chamber rod/pin mount 👍  Also assuming one may well have left or right mount slack adjusters.

Richard you can adjust them tight and back them off so not to scrape, this amount to however much you want. As long as when done adjusting and a brake application is made as in procedural guidelines the rod movement measures within DOT specifications.

I like it when manually checking rod movement with a pry tool a measurment of 1/2” - 3/4”, again this in my opinion is best done with that particular wheel off the ground and able to turn by hand.

Good day there

Floyd
Some good info here. I always use a pry tool to test for length of pull after an adjustment. The whole idea is to have balanced braking. You should be under that bus looking around, watching for brake, steering, suspension problems before they show up on the road. Every time I adjust brakes, hit all the grease fittings on the slack adjusters, S-cam bushings, and some penetrating oil on the slack adjuster clevis pin and make sure they are not frozen by turning them with vise grips, open any air tank petcocks. After all, you are servicing the brakes-one of the most important and taken for granted systems on a bus.

ps: the little plastic c clip washers for the chamber rod measure is the best thing ever, plus its a seal of sorts
Title: Re: Break slack
Post by: solardude on June 13, 2018, 03:35:32 PM
WOW! I had no idea this was such a controversial subject. Interesting to read all the different "opinions". I had not checked back on this for few days. I simply followed the guide in the manual, which calls for 2 clicks up front and 3 in back. I had always had a pull on the front, turns out after further adjusting to try to correct the pulling, and more inspection I had not noticed before that the PO had installed the incorrect lining on a single shoe causing strange breaking problems. In the end, for the rears, DOT rules are less than 1 3/4 stroke, three clicks amounts to about 3/4" travel, and 5/8" for two clicks. The drive are working good now, the tags are not quite right yet, I can see they have not been correct for a long time, the tag tires are octagon from all the skidding. More work/investigation needed for each of those. I am planning to have the fronts turned for the proper sized linings, then reevaluate the linkage on each wheel.

Bus work is always a journey, with interesting and sometimes frustrating stops along the way. For now, the bus sits till I can take the front end apart again.
Thanks for all the interesting commentary and info.

SD
Title: Re: Break slack
Post by: Jim Eh. on June 13, 2018, 06:07:13 PM
You definately cannot get brake application timing or pressure from a difference in brake adjustment.
Like Brian was said, max application should result in a 90 degree angle between pushrod and slack adjuster center line. Also at that point the pushrod should NOT be in contact with any part of the base chamber of the brake chamber. In other words the pushrod should be centered in the hole when the brakes are fully applied.

The factor that controls brake timing is crack pressure in the relay valves. On older units the relay valves may have been changed more than just a few times. If you find for instance the tag axle seems to be applying after the drive and or steer, investigate the correct brand and specs of the relay valve and change out the relay valve to the correct one. This comes with a caveate. It could actually be the drive or front axle or both have a relay valve with too low of a crack pressure and are activating too soon.

With a vehicle of this length there is a lot of lag time between application of the foot pedal and brake shoe contact with the drum. Lots of air line length to make the system work as designed. Plus air compresses so it adds additional "lag" time. It does not affect brake induced pull to one side but it may result in reduced control when applying your brakes on a slippery surface while turning. This along with chamber size and slack arm length is how manufacturers overcame understeer in a corner in slippery conditions.
Title: Re: Break slack
Post by: buswarrior on June 14, 2018, 05:48:29 AM
Skidding tag axle tires will usually be found to be too lightly loaded.

Check the real air pressure output of the regulator that feeds the tag axle air bags.

When the scale house is empty, many jurisdictions will leave the scales turned on. Roll thru there, stop each axle on the scale, write down the numbers displayed on the board, IIRC, you want those tags over 3000 lbs, depending on coach loading, more for more.

And, as always, check: did some idiot previous owner put a bigger brake chamber/longer slack adjuster on there?

Those tags didn't slide for the mainline fleets, unless something was wrong. Through maintenance, you can too! The duals blast them with crap, first clevis pins on the coach to seize, first brake chambers to fail with dirt blown into them, dirt dirt dirt, grease the slack and cam like a religion...

happy coaching!
buswarrior
Title: Re: Break slack
Post by: chessie4905 on June 15, 2018, 06:56:07 PM
Also many truck stops have Cat scales that can weigh each axle and maybe each wheel position. Or you could stop at a dot inspection site and have them do the weighing....umm, uhhh, maybe not...
Title: Re: Break slack
Post by: Melbo on June 16, 2018, 10:07:23 AM
I am kind of late to this party BUT when I got my bus about 14 years ago I took it to a national chain big truck / bus service center and had the brakes adjusted.  My tags locked up and the steers would pull when I braked.  I took it in for an alignment to a local shop.  They asked who adjusted the brakes and I told them.  They laughed and said they were not adjusted properly but they would be better now and they were.  About a year later I took it back to the local shop for king pins and a brake adjustment and asked a bunch of questions about properly adjusting the brakes.  They said the best thing to do was auto adjusters BUT the proper way to adjust them was ---- and gave me specific instructions.  My steers and tags are adjusted the same and my drives are adjusted differently.  I do it my self every year and I have never had any pull from the steers and never locked up the tags since then.  So I'm guessing that not every brake adjustment on every bus is always the same.  I check all the stuff they showed me every time I get under the bus to lube and adjust thing.

Just my way

Melbo
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