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Air Brake Problem, No Ones Ever Seen..


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I read the forums from time to time, and love looking at all the old Mack pics.. Now I need some advice on modern Macks.

I have a 2001 CH613 281k,mi. day cab with the Meritor brakes and ABS. I pulled a load Monday and after I was empty and driving home I had a hard time stopping.( trailer brakes locking up)

I took my time back to yard and unhooked drove out from trailer, only brakes that worked, fronts.. :pat:

Stopped tractor and inspected rear brakes, within adjustment but adjusted agian.. I drove off felt good for 2-3 stops, THEN ONLY fronts./?? I live 6 miles away at this point and limped home..

When I got home and asked wife to apply brakes and the seemed to apply full 2-3 times @130psi then nothing till under 100 psi and the appiled harder and harder as pressure dropped.

I broke off all service air lines to hear flow, fronts full output all the time. While the rears full at first, then nothing till 100psi or lower.

I have installed a new foot valve and did the same thing..??

Pulled fuse for ABS ign. brakes worked fine. I thought that when ABS fails you still should be able to stop..?

Anyone run into this problem??

Thanks in advance for the feed back,

Doug

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ABS when it fails it seems to fail in a most unpleasent way. I almost crashed into another car in my Chevy Tahoe after the ABS module went bad. It decided that normal braking was a skid and just let the brake pressure drop, had to apply the emergency brake to get it to stop. Pulled the fuse and never cared to fix it.

There might be some type of pressure sensor within the ABS modulator that went bad. That or the modulator computer itself is bad.

Eh I like electronics and computers but when it breaks you better have some asprin handy.

-Thad

What America needs is less bull and more Bulldog!

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I can't remember the actual name for it but I call it the bobtail valve. Some trucks don't have it, some do. When your trailer is hooked up full pressure is applied to the brakes on the tractor, when trailer is unhooked your only given about 30 to 40 percent of full air until your brake application pressure reaches 100 psi or so and then your given full air all at once. Maybe this valve is gone .

I know alot of people that have turned a road tractor into a dump truck and forget to take this valve out. Real scary when loaded for the first time and you have to stop real quick!

Hope this helps you try to locate the problem.

greg

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I can't remember the actual name for it but I call it the bobtail valve. Some trucks don't have it, some do. When your trailer is hooked up full pressure is applied to the brakes on the tractor, when trailer is unhooked your only given about 30 to 40 percent of full air until your brake application pressure reaches 100 psi or so and then your given full air all at once. Maybe this valve is gone .

I know alot of people that have turned a road tractor into a dump truck and forget to take this valve out. Real scary when loaded for the first time and you have to stop real quick!

Hope this helps you try to locate the problem.

greg

I dont think it will have a bobtail vavle if it has abs.

glenn akers

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Are you still have problems? I talked to our Wabco rep. Two things pop out.

1. Your auto slack adjuster is out of adjustment.

2. Your air compress is shot and you have sludge in the tank and valves.

Unfortunately we feel your best bet is to take it to a dealer, you could be chasing a ghost.

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had this same problem with an 05 CHN . It was a sensor on a wheel for the abs. same symptoms as your truck toa "T". We let the dealer handle it. Brad

The ABS warning light would be on if it was the sensor, he didn't mention anything about the light. Is that on too?

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Are you still have problems? I talked to our Wabco rep. Two things pop out.

1. Your auto slack adjuster is out of adjustment.

2. Your air compress is shot and you have sludge in the tank and valves.

Unfortunately we feel your best bet is to take it to a dealer, you could be chasing a ghost.

You could take it to a dealer, let them chase the ghost. Just give them a blank check, let them change parts (at list price), test drive on your fuel and time. Then go change some more parts until they get the right one. Let us know what it cost!

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Hey now,

Lets not bash on the dealership Tech, I just happen to be one. It can be a hard way to make a living!

Fred

Amen to that Fred. Think how the body shop guys feel chasing water leaks, squeaks, rattles, and inoperable electronics and wiring too!! It can be a very difficult way to earn a living!!

Rob

Dog.jpg.487f03da076af0150d2376dbd16843ed.jpgPlodding along with no job nor practical application for my existence, but still trying to fix what's broke.

 

 

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Are you still have problems? I talked to our Wabco rep. Two things pop out.

1. Your auto slack adjuster is out of adjustment.

2. Your air compress is shot and you have sludge in the tank and valves.

Unfortunately we feel your best bet is to take it to a dealer, you could be chasing a ghost.

1. Auto slacks work fine and that was the first thing I went to.

2. Air compressor is a 4cyl. 31 cfm and works great I air up tankers to off load. I perge my air tanks daily, no sludge.

After replacing foot valve, and riding aroud with ABS fuse out for a few hours, I reinstalled fuse after I got loaded, I've not had a problem soo far.

When reinstalling foot vavle we noticed that the front brakes had a larger air line then the rears. We decided to switch this around. :idunno:

Everything seems fine. On the topic of dealer, mine could not look at truck for 3-4 days. So I decided to chase the ghost. At this point it cost $262, I have not ever left the dealer for under $1000, even for an ABS sensor that shut down a truck, not this one a Vision..

Thanks for all the input,

Doug

BTW, I've had good dealing with the dealer just cost alot.

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When reinstalling foot vavle we noticed that the front brakes had a larger air line then the rears. We decided to switch this around. :idunno:

Bad idea!

The reason the front air line is thick is because the line runs right from the pedal to the brake chambers so you need allot of air flow(also helps supply the trailer and trolly brake). The rears have a relay valve that doesent need the flow of a larger line. A larger air line from the primary tank runs to the relay valves supply port then the thin line from the pedal runs to the control port. Then you have 4 delivery ports to each of the four brake chambers on the rear wheels. Your restricting air to the front brakes.

So when you pull the ABS fuse it doesent have that problem? Try driving around for a day with the fuse out or even longer. if there is no problem then its a bad ABS component. Maybe a bad ABS ECU or bad sensor.

Brad those super single tires can save something like 1000-1200 pounds on a tractor trailer if both drives and trailer wheels are super singles.

-Thad

What America needs is less bull and more Bulldog!

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who do you like those wide tires? How much wieght did ya lose with them? I have heard that your fuel milage goes up too.... is that true? Thanks, Brad

I already had aluminum rims on it only cut my weight down about 150-200 lbs.

Fuel mileage increased .5-.75 mpg, depending on load.

Bad idea!

The reason the front air line is thick is because the line runs right from the pedal to the brake chambers so you need allot of air flow(also helps supply the trailer and trolly brake). The rears have a relay valve that doesent need the flow of a larger line. A larger air line from the primary tank runs to the relay valves supply port then the thin line from the pedal runs to the control port. Then you have 4 delivery ports to each of the four brake chambers on the rear wheels. Your restricting air to the front brakes.

So when you pull the ABS fuse it doesent have that problem? Try driving around for a day with the fuse out or even longer. if there is no problem then its a bad ABS component. Maybe a bad ABS ECU or bad sensor.

Brad those super single tires can save something like 1000-1200 pounds on a tractor trailer if both drives and trailer wheels are super singles.

Guess I know what I'm switching tommorrow.. :pat:

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  • 3 weeks later...

Well the problem came back in a BAD way. Make a routine stopp for a traffic light in the rain only fronts worked. :pat: I did a 180deg stop, and almost flipped as the truck hooked hard. I was bobtailing.

Off to the dealer I went. They got it in within 8hrs and said all 4 rear slacks where bad, not racheting properly. That's the first thing I checked and my mechanic checked. I strongly disagreed and told them to check further in the air system.. Well low and behold after many hours of checking then put on the Vis machine, BAD RELAY VALVE leacking internaly.

Now that they found the problem I told them to replace slacks just in case. :D I don't want another round about ride.

So hopefully this thread will be closed. :idunno:

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  • 15 years later...
On 10/22/2006 at 4:48 PM, QCDriver said:

Hummmm, Good question.

Someone to just start replacing parts? Alot

Someone to hook up to a machine and read what's really wrong? Priceless :rolleyes:

Was your problem fixed? I’m currently having this exact issue. My brakes are locking up while I break! Mechanic has adjusted the slacks, however I do have a valve leak. 

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On 9/22/2022 at 11:53 PM, h67st said:

Sounds like his problem was a relay valve, that would be my thought if a truck was acting like this.

My experience would tell me the same thing

I have had sticking brake pedals before that took me a while to find

Kind of embarrassing really but we live and learn

Paul 

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