Check slack adjusters

Tom Y · August 07, 2011, 05:50:09 AM

Just a reminder to check brake adjustment. My brakes stop well, no problem..... I thought I had auto adjusters front and rear. Greasing the front I checked stroke, to much. No problem to adjust, but it should not have been that much. The back looks good yet. Tom Y

wal1809 · August 07, 2011, 08:28:09 AM

When I get this fuel leak fixed that will be my next project.

RoyJ · August 08, 2011, 09:43:53 PM

Commercial driving schools tell you to check once a day as part of your pre-trip.

I doubt a single trucker on this planet does that. I check my slacks before every major trip. More often when I'm in the rockies (pretty much every square inch of BC), and less so when I'm in the prairies.

Tikvah · August 09, 2011, 06:29:58 AM

Okay... I'll brave being the one to ask. How do I check my adjusters? What am I expecting to see? I can't even get under my bus, much less check anything. Show me a picture, I'm not even sure I know what a slack adjuster is or what it should look like.

I drove 40' MCI's years ago for a living, then school buses. I know we always talked about slack adjusters, and I'm sure at some time during some pre-trip training, some mechanic crawled under to show us what to look for. But that was many years ago. Now, I don't remember. Besides, I'm totally convinced that there is a major percentage of folks who are reading this forum who have NEVER driven professionally and have no idea what we're talking about.

So, I'm serious, I have no idea even how to get under my bus, much less what I would check if I could. So call me "brave" for asking, or simply stupid, but I promise that I'm not alone.

Dave

saddleup · August 09, 2011, 06:37:06 AM

Tikvah, man you are Brave, interesting to see where this one goes
Matt

buswarrior · August 09, 2011, 06:43:22 AM

Here's one resource:

http://www.triport.net/Air%20Brakes/_Commercial_Vehicles_Air_Brake_Adjustment.pdf

Measuring the stroke using air pressure is a more reliable method than the tug test, as we may not be strong enough to find that the brakes are indeed out of adjustment.

There's a lot more to discover in our old worn out coaches besides worn brakes, worn camshaft bushings will also prevent us from getting the most brake squeeze we seek in a panic stop.

There are many evenings of reading available about brakes, and well worth the time spent BEFORE you can't stop properly.

happy coaching!
buswarrior

Lin · August 09, 2011, 09:35:38 AM

I must say I have not been testing pushrod travel. Rather, I have been adjusting the brakes before trips. I, perhaps wrongly, thought that freshly adjusted brakes would naturally be in spec. Am I way off here?

Barn Owl · August 09, 2011, 09:48:31 AM

QuoteI have no idea even how to get under my bus

Dave,

That is how we all start out. This topic is important enough to be discussed often. There are always new readers that show up for the first time and they don't even know what a slack adjuster is, much less what to even search for. First thing to do is learn how to jack your bus up and properly block the airbags so you don't kill yourself. Being you have a MCI someone who knows that bus needs to post a how to for that make. If you have a manual it should also be in there. Getting under a bus without a pit is no small task. Don't want any "Squished nuts".

Tikvah · August 09, 2011, 09:55:08 AM

I've been thinking about building some kind of wooden block system to lift my bus. For both sides of the bus drive/tag wheels I'm picturing four 6x6 posts 5' long with an angle cut on one end. The four 6x6 beams would be bolted together with some threaded rod and a big wing-nut. That way I won't have one roll away and allow the bus to jump off. The rod with the wing-nut would allow them to separate, because the whole thing together would be too heavy.

I don't have any manuals, but I sure wish I did. ( I have a downloaded version of the MC-9 Crusader II Maintenance Manual, January 1989)

Dave

bevans6 · August 09, 2011, 10:35:17 AM

Here is the thing - you can't properly adjust manual slack adjusters (you should never adjust automatic ones, they either work or they need to be replaced or serviced) without doing a push-rod extension test. And the only way to do a push-rod extension test is to measure the static extension, then fully apply the brakes with the air pressure between 90 and 105 lbs, and measure again. Then you compare to the spec for your particular canister. There is no other way to do the job properly, end of story. There are lots of ways to do the job less than properly, obviously. As BW points out, you need to put full stress on the parts to observe what worn bearings, bushings, maybe a cracked mount, a loose widget are going to do. If you don't have a tool in the bus that will allow you to apply the brakes (they sell little extension pushrods at truck stops) then you stand a chance, albeit small, of failing a roadside inspection.

The pressure you can apply by the "tug test" is on the order of 30 lbs. The pressure applied by a 30 size cannister is 3,000 lbs with 100 PSI. A size 20 can like on the front of my MCI develop s 2,000 lbs of push. Which do you think is going to find a failing part?

In commercial service (up here in Ontario) a bus gets a documented under chassis inspection including measuring brake stroke every 30 days or 12,000 km. Daily inspection by the driver is not required. My compromise is to measure the stroke on the rears (which have auto slacks) at the beginning of every season, or before my first long trip. They are a total and complete PITA because the only way I can do it is to take the wheels off. The fronts are manual, and I inspect them before every long trip. Once I do that I do a tug test (which is a useful test, just not a definitive test) and I repeat that if I've driven a couple thousand miles and I have some time on my hands. I do a tug test after the proper test and adjustment so that I know exactly what to expect.

Edit: I forgot to mention that to do the fronts on my MCI all I need to do is crank the wheel so the front points out, and the brake canister is easily reachable around the front of the tire. It's right there. It's also important to ensure that you don't adjust for too little movement - I tap the brake drum to make sure it rings like a bell with the brakes off. Or I do the adjustment when i am doing an inspection and the wheel is off anyway.

Brian

Brian

buswarrior · August 09, 2011, 07:20:14 PM

Lin, without measuring pushrod stroke, how do you know the brakes are in adjustment?

If you don't measure before and after, how do you know you needed to, and how do you know you set them right?

Some who never measure after they adjust might be quite shocked at how little a change in the degree of backing off changes the applied stroke.

Some might be shocked at what they might be going down the road with after "confidently" adjusting the brakes.

happy coaching!
buswarrior

Lin · August 09, 2011, 08:01:23 PM

BW-- I adjust the brakes by tightening them until the shoes are up against the drums and then backing off between 1/4 and 1/2 turn. I had thought that if the shoes were that close to the drum, they were in adjustment. I can see that would be an issue if the shoes were getting worn down, but it seemed that with new or newish shoes, I was okay. Apparently, although this might be usually correct, it is not totally so. Is the issue that there might be play in other components?

If there is such play, then the brakes could not really be adjusted until that problem is fixed.

buswarrior · August 09, 2011, 08:16:16 PM

You've got it right Lin.

There is a rather shocking lengthening of push rod stroke for a relatively small change in the 1/4 to 1/2 turn backing off.

After you do some experimenting with the degree of backing off and measure pushrod stroke, and see the differences, you'll likely never adjust without checking stroke again. That locking nut really sucks us in to set them a little too loose at times.

Best practice is to have the wheel off the ground to allow you to give it a spin to confirm the linings are free.

And, yes, worn cam bushings will not allow the linings to be free and keep stroke within the adjustment limits.

happy coaching!
buswarrior

niles500 · August 09, 2011, 08:37:54 PM

Best to consult Da Book - lotsa break systems here - adjustments are different for drum and disc brake systems - my Prevost has two different systems and two different adjustments - What do you mean that auto slack adjusters should not be adjusted? - I thought we had already hashed that one over? - FWIW

RoyJ · August 09, 2011, 10:53:57 PM

Quote from: Tikvah on August 09, 2011, 09:55:08 AM
I've been thinking about building some kind of wooden block system to lift my bus. For both sides of the bus drive/tag wheels I'm picturing four 6x6 posts 5' long with an angle cut on one end. The four 6x6 beams would be bolted together with some threaded rod and a big wing-nut. That way I won't have one roll away and allow the bus to jump off. The rod with the wing-nut would allow them to separate, because the whole thing together would be too heavy.

I don't have any manuals, but I sure wish I did. ( I have a downloaded version of the MC-9 Crusader II Maintenance Manual, January 1989)

Dave

If you try 6x6s, cut a very shallow angle, or else you won't be able to make it.

I basically cut the full length of a 14" chainsaw, and even with 6:1 reverse gear ratio and 4.10s, my bus won't make it up the 6x6s now matter how hard I tried (short of burning up the clutch).