Break slack

[eagle19952]

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.

[DoubleEagle]

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.

[eagle19952]

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 ....

[buswarrior]

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

[luvrbus]

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     

[chessie4905]

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..

[richard5933]

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.

[eagle19952]

Just because the splines match on the slack and the S cam doesn't mean you have the OEM spec'd slack.

[brmax]

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

[Lee Bradley]

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.

[bevans6]

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.

[buswarrior]

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

[Friday1]

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

[solardude]

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

[Jim Eh.]

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.