Will this matter?
#41
09-23-2007, 04:06 AM
Originally Posted by Ridge Runner
Originally Posted by Sp33dbump
Great Discussing and Info everyone!....Thank for the read!
Yes it is!!! I nschool we were taught to get the frame under the trailer, hook up air lines, check height of trailer, THEN back on under the trailer. I too ask why. I was also told about the OLD trailers and their braking system. :roll: I can't believe that wasn't out-dated with the invention of the wheel. As far as dropping trailers: As a driver that drops and hooks 9 to 10 times a night I have to say that if you have air bags then crank the landing gear to about 1/2 inch for loaded trailers and 1 to 2 inches for empty trailers. EVERYONE cad get under them and crank them ( the landing gear ) up MUCH easier. I can't believe any trailers that old are still in use! However, based on the age of some of the tractors I've seen..... :roll: Speaking of air bags, I always crank the dolleys until I HEAR the hiss of the airbags that indicates the weight has been lifted from them, then about 2 or 3 more cranks.
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#42
09-23-2007, 04:22 AM
Those old (pre FMVSS 121) trailer brakes were just as stated:
No spring actuated parking brakes. When you shut off air to the trailer or pull off the red gladhand, the trailer air valve would send full air pressure from the trailer air tank to the brake chambers on the trailer wheels. This worked fine as long as there were no air leaks in the trailer brake system. Once the trailer air tank ran out of air? No brakes on the trailer whatsoever, until you hooked up the supply line and charged it up again. That is why you would get close, hook up the air lines and charge the trailer with air, and then apply the trailer hand valve before backing under one of those. It's not B.S., I hauled heavy equipment on a trailer set up that way back in the late 70's early 80's.
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If you can't shift it smoothly, you shouldn't be driving it.
#43
09-23-2007, 05:09 AM
Originally Posted by golfhobo
ALL "air brake systems" use a holding tank for the air, which is resupplied by the tractor air compressor. While these tanks are full of air, the air can be released to RESTRICT the brakes. When you pull the red valve, it depletes all the air in the lines AND in the tanks! Therefore, with no air available to restrict them, the SPRINGS cause the brake shoes to CLAMP down on the drums, stopping the truck or trailer.  It's really very simple, the older style of trailer brakes basically only had a service brake, that needs air to work. Newer brake chambers have a service brake and a spring brake, your steer Axel uses service only type chamber, your drive Axel's use a service/spring type chamber. ( much bigger chamber ) Air pressure is needed to "cage" the spring to release the brakes. When you pull the red button your dumping only the air caging the spring, reservoir air is still maintained in the tanks. I could explain more about air leaks and how the system slowly applies the springs, but I don't want to give anyone a headache :wink: When you hook onto a trailer, look at the brake chambers to see if they have spring-brakes, not everyone is pulling newer trailers. Using a cage bolt is not that rare, and everyone should know how and why they are used, they are mounted on all brake chambers for a reason.
#44
09-23-2007, 05:14 AM
golfhobo,
Sorry that we got you so confused but for the most part you need not worry about the older system as you will never in your lifetime have to tow one. To ease your confusion in the mean time let me just say that the old system worked the way it did because of the way that the valves were plumbed so that the service brakes were applied when the red knob was pulled. Modern trailers exhaust the air from the spring pots and the springs apply the parking brakes. If you ever try to back under a trailer and it rolls backward then keep this discussion in mind, stop and hook up the hoses and apply the trailer brake. Then the trailer will stay put. If it still rolls then check to see if the brakes are caged and don't try to pull it if they are. Find out why they were caged. For now, If your company is ok with not attaching the air hoses then don't bother. But if you're in this business for long enough at some time you will sign on with a company that will require hoses attached before hooking and during drops. You have to decide if you follow this policy or not and face the consquenses in the event of a failed hook or drop should you not follow it. It really all boils down to trainers and instructors that were not taught correctly that are now teaching incorrectly to the new generation of drivers who will pass along this incorrectness to future drivers. And it just snowballs from there. Have a good week and be safe above all, Mad Fatboy
#45
09-23-2007, 05:25 AM
Originally Posted by Splitshifter
Those old (pre FMVSS 121) trailer brakes were just as stated:
Especially towing an empty 121 trailer on a slippery road with a non-121 tractor and trying to stop for a light. It jack-knifed me in the middle of the intersection. Luckily I was going so slow that there was no damage. Mad Fatboy
#46
09-23-2007, 05:40 AM
Originally Posted by Mad Fatboy
Originally Posted by Splitshifter
Those old (pre FMVSS 121) trailer brakes were just as stated:
Especially towing an empty 121 trailer on a slippery road with a non-121 tractor and trying to stop for a light. It jack-knifed me in the middle of the intersection. Luckily I was going so slow that there was no damage. Mad Fatboy We usually removed the so called "anti lock" relay valves and replaced them with standard relay valves. At least then you knew you could stop if you had to!!!
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If you can't shift it smoothly, you shouldn't be driving it.
#47
09-23-2007, 05:52 AM
Originally Posted by COLT
Originally Posted by golfhobo
ALL "air brake systems" use a holding tank for the air, which is resupplied by the tractor air compressor. While these tanks are full of air, the air can be released to RESTRICT the brakes. When you pull the red valve, it depletes all the air in the lines AND in the tanks! Therefore, with no air available to restrict them, the SPRINGS cause the brake shoes to CLAMP down on the drums, stopping the truck or trailer. :lol: :lol: Yeah.... ME TOO!! :wink: It's really very simple, the older style of trailer brakes basically only had a service brake, that needs air to work. Newer brake chambers have a service brake and a spring brake, your steer Axel uses service only type chamber, your drive Axel's use a service/spring type chamber. ( much bigger chamber ) Air pressure is needed to "cage" the spring to release the brakes. When you pull the red button your dumping only the air caging the spring, reservoir air is still maintained in the tanks. Okay, you may be right about this, it's been awhile since I studied this stuff. But, this doesn't sound quite right to me. I know when you APPLY the service brakes, it releases only the chambered air to momentarily apply the brakes, and it is quickly resupplied from the tanks when you lift the brake pedal. (you would only hear a small hiss if you were standing back there.) But, when you pull the valve to set the brakes, you hear a loud, longer hiss. I was under the impression that this was because you had dumped all the reservoir air as well, and explains why it takes a few seconds or more to release them once you pushed in the supply valve again. You are having to "recharge" the entire system with air again, not just the chambers. :?: I could explain more about air leaks and how the system slowly applies the springs, but I don't want to give anyone a headache :wink: When you hook onto a trailer, look at the brake chambers to see if they have spring-brakes, not everyone is pulling newer trailers. That is probably a good idea for many drivers who are constantly switching trailers. My company only has a certain number of trailers, and all of them have spring brakes. I use one trailer per weekly run. Using a cage bolt is not that rare, and everyone should know how and why they are used, they are mounted on all brake chambers for a reason. I can see how this might happen, and I think I DO remember being shown them in school. I just didn't remember it, nor did I recognize the terminology. It sounded like something out of the past!
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Remember... friends are few and far between. TRUCKIN' AIN'T FOR WUSSES!!! "I am willing to admit that I was wrong." The Rev.
#48
09-23-2007, 06:00 AM
Okay, you may be right about this, it's been awhile since I studied this stuff. But, this doesn't sound quite right to me. I know when you APPLY the service brakes, it releases only the chambered air to momentarily apply the brakes, and it is quickly resupplied from the tanks when you lift the brake pedal. (you would only hear a small hiss if you were standing back there.) But, when you pull the valve to set the brakes, you hear a loud, longer hiss. I was under the impression that this was because you had dumped all the reservoir air as well, and explains why it takes a few seconds or more to release them once you pushed in the supply valve again. You are having to "recharge" the entire system with air again, not just the chambers.
When you apply the service brakes, the signal line from the foot valve sends the application pressure signal to the relay valves on the tractor brakes and to the trailer valve, these valves in turn supply the required amount of air pressure to the service side of the brake chambers, thereby applying the brakes. Applying the service brakes has nothing to do with exhausting air from the spring brakes. (Well there is one small exception in the case of the anti compounding valve which prevents damage to the brake mechanisms in the event that the service brakes are applied with the parking brakes already on, but this has nothing to do with normal service brake operation.)
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If you can't shift it smoothly, you shouldn't be driving it.
#49
09-23-2007, 06:50 AM
Originally Posted by Splitshifter
Okay, you may be right about this, it's been awhile since I studied this stuff. But, this doesn't sound quite right to me. I know when you APPLY the service brakes, it releases only the chambered air to momentarily apply the brakes, and it is quickly resupplied from the tanks when you lift the brake pedal. (you would only hear a small hiss if you were standing back there.) But, when you pull the valve to set the brakes, you hear a loud, longer hiss. I was under the impression that this was because you had dumped all the reservoir air as well, and explains why it takes a few seconds or more to release them once you pushed in the supply valve again. You are having to "recharge" the entire system with air again, not just the chambers.
When you apply the service brakes, the signal line from the foot valve sends the application pressure signal to the relay valves on the tractor brakes and to the trailer valve, these valves in turn supply the required amount of air pressure to the service side of the brake chambers, thereby applying the brakes. Applying the service brakes has nothing to do with exhausting air from the spring brakes. (Well there is one small exception in the case of the anti compounding valve which prevents damage to the brake mechanisms in the event that the service brakes are applied with the parking brakes already on, but this has nothing to do with normal service brake operation.) From Wiki:
The brakes are applied by pushing down on the brake pedal (also called the foot valve or treadle valve). The further you push down on the pedal, the more air pressure is applied to the service brake actuators or brake chambers. Letting up on the brake pedal releases this air pressure and hence releases the brakes. It is a common misconception to believe that the service brakes operate by releasing air pressure and relying upon the spring brakes for appication but this is not the case, as the spring brakes and service brakes operate on separate chambers within the booster assemblies.
If the pressure in the emergency line drops, due to the activation of a valve in the cab, the disconnection of the emergency line coupling, or a break in the emergency line, the spring brakes will come on, because there is no more air pressure in the trailer air tank to hold them back.
Or is it that, when you pull the valve, you are only "expressing" the air in the LINE so that there is no longer any pressure on the air in the reservoir tanks? I suppose this could account for the long, loud squeal/hiss I hear, but when I "resupply" the air, it sure SOUNDS like I can hear AIR being forced into the reservoir tanks (when standing back there.) Okay.... now I've REALLY got a headache! :lol:
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Remember... friends are few and far between. TRUCKIN' AIN'T FOR WUSSES!!! "I am willing to admit that I was wrong." The Rev.
#50
09-23-2007, 07:17 AM
When you pull the red knob, it shuts off air to the trailer supply line and exhausts the air pressure from the supply line.
In addition to the "red" line being a supply line, it is also a signal to the trailer valve for the purpose of parking brake application. When the trailer valve is no longer getting pressure from the supply line, it exhausts the air from the trailer spring brakes causing the brakes to be applied. At the same time, the trailer valve prevents pressure from the trailer air tank from leaking back thru the supply line. The trailer tank stays fully charged, even though the pressure is exhausted from the spring brakes. In the case of an old trailer without spring brakes, when the red knob is pulled, the pressure is shut off to the trailer, air is exhausted from the supply line, and the trailer service brakes are applied, using air from the trailer air tank. The trailer air valve also prevents air from the trailer air tank from leaking back thru the supply line. The old type trailers used a different type of trailer valve of course.
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