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Author Topic: Spring brakes won't hold  (Read 2674 times)
glang
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1998 Prevost XLT




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« on: August 13, 2006, 03:41:11 PM »

Working on a Prevost Lemirage. Parking brake will not hold even on a slight grade. I have adjusted slack adjusters correctly, cleaned and greased all working brake parts , replaced the cans (spring brake chambers)with new ones, replaced the QC-1C valve.I also checked for air still in the spring brake chamber but there is none  when the front valve is pulled to exaust, which is the way it should be.. Regular braking seems fine . Brake shoes and drums seem ok Never ran into this problem before. Anyone with ideas for a fix.

gary
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Dallas
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« Reply #1 on: August 13, 2006, 04:39:41 PM »

Gary,

Here's a thought:
Make sure that the rollers you are using in the S cam are the correct ones for that application. If by some chance someone replaced the large rollers that probably came on the bus with the newer smaller ones, and didn't replace the S cams at the same time, you won't be able to get a full application of spring brake pressure. (Smaller rollers would also prevent the S cam from camming over).
Another thought would be to measure the interior circumference of the brake drum itself. If, by some chance someone turned the drum beyond the ability of the S cam to expand on application of the parking brake, it won't get a full application of spring pressure.
The reason the service brakes work is that you can apply more pressure with them to get more extension of the brake shoes. The spring brakes can only extend so far before the spring is at it's maximum length.

I'm probably wrong, but, hey, it's worth looking at, and it sounds logical in my mind.

Good Luck!

Dallas
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NJT5047
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« Reply #2 on: August 13, 2006, 07:04:31 PM »

Dallas has covered this, and he's probably solved the problem if the braking problem started before the chambers were removed...but I'm gonna have a guess too. Did this problem present before brake work? Or is this the outcome of a brake job or other work on the brakes?
Are the drive axle chambers of original design..or at the least identical to what you removed?  Is it possible that the chambers are not correct for the application?  Spring brakes are pretty simple...problem should be apparent to a truck brake man...and Dallas. 
Have the drive axle drums been checked for cracks? 
Have you measured the distance of travel (articulation) of the slack ajusters when the service brake is applied compared to the park brake?  May rule out the chambers.   The "normal" braking action when using the service brakes may be the steering and tag brakes functioning well, with the drive axle still incorrect...but a lightly loaded coach may feel normal. 
Do Prevosts have OEM spring brakes?   I learn something every day....thought most late model buses were fitted with DD-3s. 
Post what you find...be interesting to know what's causing your problem.   One would think that either can would hold the bus...? Seems odd that both drive axle brakes would cause problems at the same time. 
Best, JR
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JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.”

Ayn Rand
pvcces
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« Reply #3 on: August 13, 2006, 07:41:09 PM »

Gary, short of a broken spring, I think spring brakes are pretty foolproof. You should have eliminated any chance of a broken spring. This leaves me with the idea that you have a foundation brake problem. The only thing that comes to mind is the possibility of the adjusters having been turned the wrong way when they were adjusted. This assumes that there is nothing wrong with the brakes, themselves.

I hope one of us has helped you.

Tom Caffrey
Suncatcher
Ketchikan, Alaska
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Tom Caffrey PD4106-2576
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NJT5047
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« Reply #4 on: August 13, 2006, 08:31:50 PM »

One other thought.  To verify that the drive axle brakes are working with the others, measure the drum temp with an infrared gun and see how the drums compare.  This may show the drive axle drums cooler than the others..or may show the same.   Really sounds like the spring brake chambers are not correct for the application. If the brakes apply, the spring should hold whenever the air is exhausted from the chamber.  The slack adjuster should move the same amount on park or service brake application.   
I'd bet a very small amount of money that the drive axle service brakes are not working correctly either.    
JR
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JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.”

Ayn Rand
glang
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1998 Prevost XLT




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« Reply #5 on: August 14, 2006, 04:25:37 AM »

THe problem was there before I  did any brake work. Here is a thought. I took a six speed standard out of this bus and installed a 740 auto. Maybe with the standard no one noticed that the parking brake was not working as they would have left the coach in gear. With the auto we rely on the spring brake 100%. I will keep you posted on progress. Thanks for Ideas


Gary
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prevosman
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« Reply #6 on: August 14, 2006, 05:55:07 AM »

Gary,

I had a 40 foot XL and its parking brakes under the best of conditions were marginal. If the spring broke they were virtually useless. I assume you know the springs are not broken and have not compressed over time.

I would also make certain you have the correct brakes. The drive axle brakes are 30-36.

FWIW, coaches after a certain year (mid nineties I think) added spring brakes to the tag as well and that made a world of difference. I think the brakes for the tag are 16-24. If you have room it might be worth the effort to add them.
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Jon Wehrenberg
Knoxville TN
1997 Prevost Liberty
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« Reply #7 on: August 14, 2006, 08:15:55 AM »

THe problem was there before I  did any brake work. Here is a thought. I took a six speed standard out of this bus and installed a 740 auto. Maybe with the standard no one noticed that the parking brake was not working as they would have left the coach in gear. With the auto we rely on the spring brake 100%. I will keep you posted on progress. Thanks for Ideas


"PLEASE"   don't park yout standard shift bus in gear.
It is a vary dangerous practice. when you shut the engine down, the shut down on the govoner goes back to the run position, so the engine only has to be turned to have 1 piston come over the top, and the engine could start.  If the brakes are not in good condition, the vechicle can go forward, or back, which ever gear you have left it in.

I lost a Snap-on cabinat as a fellow across from me was going to tune-up a detroit, and clicked the starter over.  The engine started, and ran up on top of my tool box, and hit the truck I was working on, to stop it.    No one hurt, but the box, just wasn't the same after that.
Working on large farm equipment, it is a habit to walk up to the side of some tractors, and short across the + to the solenoid wire to start the tractor, and warm it up.   Bad habit,  unless you don't mind bar tread tractor tracks from your feet to your head.

Please don't leave your standard shift, diesel vechicle in gear!!!!!!
Thanks
Steve
« Last Edit: August 16, 2006, 05:26:18 PM by DrivingMissLazy » Logged
prevost82
82 Prevost 8V92ta 6 speed
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82 Prevost Marathon XL




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« Reply #8 on: August 14, 2006, 08:47:42 AM »

The spring brakes on my 40 ft XL seem to hold fine. I had to replace them because of broken springs but once that was done they worked good. I agree with rauchy, putting a set on the tag would be nice....will have to look into that.
Ron
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prevosman
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« Reply #9 on: August 14, 2006, 11:55:22 AM »

I just looked at some photos I had of my 1987 XL and while it isn't conclusive, it looks like a spring brake is physically possible to be installed. Maybe someone can verify the space is available, but since my 97 has spring brakes on the tag and drive, I never want to go back to a coach with single axle spring brakes.
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Jon Wehrenberg
Knoxville TN
1997 Prevost Liberty
Sammy
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« Reply #10 on: August 14, 2006, 02:02:01 PM »

I'd measure the drums and the linings as Dallas already suggested.
If you don't have the proper drum guage, take them to a truck repair shop or parts house to have them measured.
Best of luck with your repair.  Cool
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glang
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« Reply #11 on: August 14, 2006, 03:32:10 PM »

.

The spring brakes on my own Prevost work fine. I have parked on steep grades and even with my loaded 25' trailer on behind they held firm. However, I had done a major brake job a couple years ago, new everything on all six wheels.

 Today, I disassembled both drive wheel brake @$#'y and ordered new slack adjusters, rollers, springs and S cams from Prevost.
The drivers side S cam was severly pitted and  had not seen grease for years as the grease fitting was behind the slack adjuster and impossible to reach.(Had to relocate it)
Drums are well within spec for wear, pads are good. I had already installed new 36 cans(chambers) Saturday. Slack adjusters were sloppy on spline. Will let you know  tomorrow night if things work OK with all new parts. Customer wanted to save money or I would have replace all these parts last week.

Thanks for suggestions
Gary L
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buswarrior
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« Reply #12 on: August 14, 2006, 03:54:04 PM »

May I re-emphasize what JR posted and correct Dallas?

A brake chamber pushrod should stroke the same distance regardless of whether the rod is being pushed using air pressure by the brake pedal, or using spring pressure by applying the parking brake control.

It matters not "who" does the pushing, once the linings are against the drum, that's it.

The issue at that point might be who is stronger, air or spring, but they can only move the brake linkage until the linings are against the drum. No further. Air will squeeze harder, but that will not show in pushrod stroke.

Assuming the brakes are adjusted, I would predict weak or broken springs.

Even a broken spring will make the brake linkage move, since you still have a lot of metal spring bits jammed in there that want to extend, however, it won't be making enough squeeze to work properly.

Spring brakes don't last forever, it is a tough life for them being caged up tight when we go for a drive, and only allowed to partially extend in order to still put pressure on the brake linkage. In their movement, the non-pressure side of the chamber must be vented to the outside, allowing water/moisture/salt/bugs to get inside, all of which are bad for the steel of the spring.

30-30 chambers are cheap, under $75 each CDN, gotta be cheaper in the US of A. I would change them out as a preventive maintenance item. It isn't worth even thinking about, just do it.

Don't trust the previous owner! Be sure that the rods on your existing ones are cut to the proper length if you are going to use them as samples for the new ones. When the brakes are properly adjusted (somewhere around 1 inch of stroke when applied) the relationship between the slack adjuster and the pushrod should be closing on 90 degrees. Don't want less than 90 when it gets out towards the 2 inch legal limit.

happy coaching!
buswarrior
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Clarke Echols
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« Reply #13 on: August 14, 2006, 05:12:12 PM »


Standard pre-trip brake inspection methods should isolate the problem.  Use this procedure:

With the bus parked on  a level spot, block wheels behind and in front of tires to prevent rolling.
With engine running, transmission in neutral, (or with engine off and air tanks at full pressure), have
someone in the driver's seat to operate the brakes.

(1) Release parking brake by moving the yellow handle to the "released" position.

(2) While watching the spring brake and slack adjuster under the bus (you need to have the
bus up on blocks or have a pit underneath so you can observe what's going on in order to do
this), have your assistant apply and hold the service brakes by pressing down on the brake
pedal/treadle.  The angle between the actuator shaft centerline and the centerline of the slack
adjuster should be close to 90 degrees (right angle) or slightly more -- perhaps 100 degrees
(maximum).  If the angle is less than 90 degrees, the slack adjusters are too loose.

(3) Release the service brakes.  The actuator shaft should recede into the brake can and the
angle between actuator and slack adjuster should be well over 90 degrees.

(4) Apply the parking brake by moving the yellow handle to the "applied" position.  The angle
between slack adjuster and actuator shaft should return to the same position as when the
service brakes were applied.  If it does not extend out to the same (90 degrees) angle, the
spring  in the brake cannister isn't applying its force properly.

If these conditions aren't met and the problem isn't the spring, you probably have a foundation
brake problem.

Clarke Echols
"Jen-yoo-wine DOT-certified, qualified air-brake inspector"  (Take that and three
bucks and you can buy a Big Mac. Smiley
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buswarrior
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« Reply #14 on: August 15, 2006, 06:32:58 PM »

Hello Clarke.

DANGER!!!

The angle of the slack adjuster to the pushrod is ENTIRELY dependant on whether the last mechanic to install the chamber cut the pushrod to the proper length, and installed the clevis properly.

Correct, that the brake chamber makes its best power when the angle is close to the 90 degrees, but that is an install issue, not an adjustment issue. Historical misconception.

Measuring applied pushrod stroke is the only reliable method to be sure you have adjusted brakes.

I'm not being a smarty pants, but when did you do a CVSA recert last?

Using the "angle" method, we need to wonder what is the varience in rod stroke, say between 87 degress and 93 degrees? What is our margin for error if we eye ball the angle a little off?

Eye balling the angle of brake linkage is an inaccurate science. When you figure the difference between well adjusted and out of adjustment on a 30-30 rear brake is about an inch, we cannot afford a lot of wiggle room! (Never mind for the rest of the chambers, where there is less than inch between adjusted and unsafe!)

To your credit, the "angle of pushrod" method was put in print in a number of credible places from the past, and was taught to a lot of drivers, however, you will have a hard time finding anyone recommend that method in print in 2006, and the enforcement community, consistantly, and without exception, will measure the actual stroke of the pushrod.

And, please, NOBODY, go under your vehicle, or let anyone else, while the engine is running.

happy coaching!
buswarrior
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Frozen North, Greater Toronto Area
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