Author Topic: How long should your air last?  (Read 13153 times)

Offline Cary and Don

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Re: How long should your air last?
« Reply #30 on: April 11, 2013, 05:06:50 PM »
There is so much information out there on air brakes,  you can find it on your own.  Bendix has tons of information.  Anybody that has air brakes needs to learn about them and exactly how they work and how to check them for proper operation.  You need to know all about the complete air system and how it works.

As for the DOT test.  We do it when on the road.  We find it amazing how much air you are actually allowed to loose and still be legal.  One thing we learned early on is you can pass that test and still be loosing air like crazy.  If the engine is running and the compressor is running,  if somebody is outside the bus,  they can hear a lot of leaks yet the air gauge says all is good.  We bought a Neoplan that was leaking like a sieve and still it had passed that DOT test before being driven here.

Don and Cary
1973 05 Eagle
Neoplan AN340

Offline belfert

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Re: How long should your air last?
« Reply #31 on: April 11, 2013, 05:37:50 PM »
There used to be a course that a driver could take that would license him to adjust manual slack adjusters on commercial vehicles in Ontario, but while the license still exists they don't offer the course anymore.  Everything is auto slack adjusters now, and you have to be a licensed truck mechanic to adjust those.  Drivers can check them, but cannot adjust them.

Is this only a law in Canada, or the USA too?  I know the general rule of thumb is that a working auto slack should never need adjustment.  If it needs adjustment something is probably wrong.

I've had problems in recent years with automatic slack adjusters and brake chambers failing.  Some of the stuff has failed after having a shop spend several hours checking the brake system from top to bottom.  I took the Bendix air brake course and understand how the system works from the compressor all the way to the brake ends, but they didn't cover replacing brake chambers, slack adjusters, and things like that.  Physically replacing a brake chamber is easy.  The bigger issue is knowing how long to cut the push rod and how to get the brakes back into adjustment after replacing the chamber.  Bus Warrior has mentioned multiple time not to just cut the push road the same as it was as the last guy might have cut it the wrong length.

I would like to replace my own brake chambers, but I don't feel I have the know how to do the job properly.
Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN

Offline HB of CJ

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Depends Upon The Condition Of The Air System
« Reply #32 on: April 21, 2013, 04:34:53 PM »
My old '74 Crown 10-wheeler would hold about 90 pounds for at least 7 days, but then it seemed a lot of stuff on the old girl had been replaced before I bought her 10 years ago.  Seems the air door can be a source of many leaks; also the brake treddel valve.  Sadly sold her.  HB of CJ (old coot) I'm back.  Vacation.  Nice. :)

Offline akroyaleagle

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Re: How long should your air last?
« Reply #33 on: April 22, 2013, 03:08:29 PM »
Here's a long explanation of how air brakes work.
I did not write it but I like it.
I do a lot of research (Messing around on line?) and I save and archive the interesting
stuff I find.


In The ShopThe Antique Truck Club of America, Inc.

          In The Shop
      Sometimes I think I’m my own worst enemy. In fact I know I am! After
      seeing my last column in print I realized that I set myself up again. Air
      brakes … what was I thinking of; Noooo.
      First, air brakes are a tough subject. Either people are totally mystified
      by them or the knowledge they have is somehow flawed. Second, do to
      federal laws; we now have two types of basic air systems being used; pre
      and post 121. Third, with all of the "voodoo changes" and "modifications"
      done by the various owners of our iron; how does one try to get this all
      sorted out? Fourth, my stack of brake information fills 14 binders and a
      stack of paper 4 foot high. What do I use and what do I ignore??? This is
      the hole I dug for myself. Smart! Real Smart… Quit laughing Greg…
      George Westinghouse invented air-operated brakes for railroad use in 1869.
      The first air brake system for trucks came in 1919. George Lane developed
      the Lane Air Brake System for logging trucks in the Northwest. It was a
      major improvement over currently available systems. It used an accumulator
      to store combustion gasses instead of a compressor. Throughout the 1920’s
      Westinghouse Automotive Air Brake Co. developed a reciprocating compressor
      and other air system control and safety components. Sold to Vincent Bendix
      in 1930 the "BW" brand continued development of relay, foot, emergency,
      relay governors, trailer air system and controls. Two more major events
      happened in the 1930s to aid the development of air brakes. Timken-Detroit
      Axle introduced the first tapered lining for heavy-duty brakes in 1935 and
      in 1938 they introduced the "P" series or ‘S" cam type of brake actuation.
      By the 1940’s, the basics of the modern air brake system were in place.
      Many refinements and improvements have taken place over the years. Mainly
      driven by state or federal laws, or in some instances by serious
      accidents, such as one in 1955 that caused the ICC (Interstate Commerce
      Commission) to mandate tractor protection valves on all combination
      vehicles in interstate service. The first state or federal law came in
      1933. The State of Rhode Island required air-braked vehicles to stop in 50
      feet from a speed of 20 MPH. This law also required all vehicle have an
      independent hand-operated emergency brake. This law required major
      improvements in braking at the time. Throughout the 50s, 60s and early 70s
      improvements, refinements and new components were introduced and
      integrated into the brake system. In 1970 Ford motor Co. introduced the
      first split braking system for air brakes with the introduction of the new
      L Series truck line. This major innovation would be mandated in 1976 by
      Federal Motor Vehicle Safety Standard 121 (FMVSS 121). Our iron now is
      legal with early antilock and split brake systems. We have come a long
      To truly understand air brakes you need to fully understand the term
      "Naugahyde Factor". To quote from a C W McCall song " it’s sorta like
      steppin on a plum Earl". By now I am sure some veterans of the windshield
      university school of driving know exactly where I’m going. The official
      definition of the "Naugahyde Factor" is as follows. The amount of grip
      generated on the seat upholstery by the drivers posterior in direct
      proportion to the amount of loss of air brake effectiveness. To fully
      understand the Naugahyde Factor you need to have three more miles of hill,
      smoking brakes and the low air buzzer going off. There have been some of
      us who have gotten out of the cab dragging the seat and floor mat with us.
      A true Naugahyde rating of 1000. After that, nothing will faze you…
      I’m going to use my usual format of fundamentals first, and then we’ll
      look at the troubleshooting as we go along. I’m going to try something
      different for illustrations though. One of the pages in this column will
      be a blank diagram you can copy and fill in as we go. Don’t mess up your
      book; copy it (the drawing) and make a few extra copies just in case. Hit
      the kids (or grandkids) crayon box for Blue, Red, Green, Orange, and
      Black. Personally I like to use Crayola colored pencils. In case the
      crying gets too loud or you don’t have leg-climbers around, hit the "big
      box stores" (Wal-Mart) etc. I know this will help you understand how the
      subsystems are put together, and figure out where the trouble is when it
      ain’t right. Yea I just said subsystems. Don’t you just hate when I say
      things like that? Coffee ready? ChCh chuuu.
      Air Brakes Part 1
      You just know I have some fundamentals that we have to deal with first.
      Without these basic principles, the nightmare would be complete. All air
      brake configurations, even the most complex start with the following.
      The energy used in an air brake system is a volume of air under pressure.
      The fact that it is under pressure is more important than volume. The
      system operates by metering amounts of pressure from a reservoir to where
      it is required. A dash gauge won’t tell you how much air is in the
      reservoir or the weight, only how tightly it is packed in. A reservoir
      weighing 30 lbs. empty will weigh just about the same with 125 lbs. of air
      pressure inside. It will make no difference if different size reservoirs
      are plumbed together at the same pressure. Volume of air only relates to
      the amount of air supply you have in reserve.
      A quick primer on reservoir and pressure will help. One hundred (100)
      pounds of pressure simply means that one hundred (100) pounds of force is
      exerted on every square inch of the confining surface. This force will
      only change if the volume (size) of the reservoir or pressure changes. If
      we have a constant pressure and volume and we compared a steel reservoir
      to a balloon, we would find the pressure in the balloon would be lower.
      Because it expands, the increase in surface area will reduce the force per
      square inch.
      If I were to connect a reservoir to a brake chamber and install an on-off
      inlet valve and an on-off exhaust valve (your foot valve), I would have
      just created a simple air brake system. I could write 17 paragraphs to
      describe the following action. Here’s the short version. When you step on
      the foot valve, the exhaust valve is closed then the inlet valve is
      opened. A volume of air now flows through the lines to a brake chamber and
      begins to push on a diaphragm that rests on a plate connected to a push
      rod. The air pressure now has more space to occupy thus reducing the
      pressure, although it will be equalized through out this basic system. How
      much strictly depends on reservoir size and the amount of room added by
      the interior space of the lines, valves and chamber. More added space
      equals a bigger pressure drop. By design truck reservoirs are considerably
      larger than the total volume of all the lines, valves and chamber interior
      areas. With this fact in mind I hope you can see how when the foot valve
      is opened the actual change in system volume is very small so the
      resulting pressure drop is negligible. If you hold the foot valve down
      long enough the pressure will equalize all over the system.
      At this point I better talk about the brake chamber and the pounds per
      square inch of force generated here. If I take the total effective area of
      a piston or diaphragm and multiply it by the applied air pressure, I will
      get the output force generated. Truck diaphragms, are sized by a type
      number. (i.e. 24, 15, 30) Doing the math gets these results. An
      application pressure of 100 PSI applied to a diaphragm of 30 sq, in.
      generates a force of 3000 pounds of force on chamber push rod. To get the
      chamber to release, we must first close the inlet valve to prevent dumping
      the reservoirs then opening the exhaust valve to release the air trapped
      in the lines and chamber. As you will learn in future columns, the foot
      application valve sequences the action of the inlet and exhaust valves for
      us when the brake treadle is pushed. Okay… at least it is supposed to.
      Another requirement of an air brake system is Pneumatic Balance. Unlike
      hydraulic brakes, when the pedal is pushed things happen almost
      instantaneously. Air brake seem to work that way but in fact they work
      slower, tenths of a second slower. This happens because it takes time for
      air to move and equalize. The chamber nearest the brake valve will
      activate the quickest and the others will follow in relationship to the
      distance from the application valve. In time all chambers will reach equal
      pressure. The same thing happens when you release the brakes. The nearest
      chamber releases first then the rest in order of distance. As you may well
      imagine, this can and will cause a host of serious braking concerns such
      as skidding, jack-knifing or bogie-hop.
      Torque Balance is the system balance that the correct axle is applied
      first, and the braking efforts are distributed equally side to side.
      Normally the pressure is delivered to the front brake chambers directly
      from the brake valve only. In order to have the rear brakes or trailer
      brakes apply slightly ahead of the front axle. This is done in hundredths
      of a second by relay valves, which are installed into the system. By tying
      the air reservoirs directly to the relay valve and then operating it with
      the foot valve the balance is achieved. I like thinking of relay valves as
      a "drivers remote foot", or an air operated foot valve. This enables the
      driver to stop the truck or tractor-trailer smoothly and safely.
      These basic fundamentals are incorporated in every air brake system. The
      engineers have to know the operational speeds, port sizes, tubing run
      lengths, diaphragm sizes, and capacities of every component in the trucks
      air brake system to achieve pneumatic and torque balance. To sum it up, we
        An Air source
        Correct Size and Number of Reservoirs
        Sequenced Valving
        Pneumatic Balance
        Torque Balance
      In the real world our iron gets "modified" during its working life and
      then by us in our restoration efforts. Sometimes we have to modify air
      brake systems because of obsolescence or we’re changing the basic function
      of the chassis from as built. There is usually enough remaining of the
      original system to keep us out of trouble. Surrrre Doug… all the time.
      Have you ever seen diagrams like these on a shop wall or in a service
      manual? One is for Pre 121 systems; the other is for 121 systems. The
      first things techs and most people say is that it doesn’t look anything
      like my irons air system. In fact it won’t match exactly, each
      manufacturer has their own routing and components, but the basic layout is
      as shown on these charts.
      We will break these complex diagrams into the subsystems that really make
      up the air brake system. One note of caution though, there is differences
      between a straight truck and a tractor or truck tractor. They are minor
      but important differences. I’ll get you to the point of understanding
      these schematics or the ones in your service manuals. Scouts honor…
      Pre 121 Tractor

      121 Truck-Tractor
      In the upcoming columns I’ll get into the four subsystems. They are as
        Air supply system
        Air delivery system
        Parking and emergency system
        Tractor system
      The first will require a whole column, as it causes the most confusion.
      (that is where most money is spent ) and the last will cover what’s
      different about a tractor or if you want to add a tow package to your
      chassis. We will look at components, how the basic ones work, how they are
      plumbed, and what happens when they oops. You’ll follow along on your own
      Time to hit the trail. I have a ton of marker light signals to give out
      for this column and the ones coming. Thanks to Knorr-Bremse (Bendix),
      Arvin Meritor (Rockwell), Haldex North America (Haldex Midland Grey-Rock),
      Ford Motor Co., General Motors Corp., Freightliner LLC, Volvo North
      America (Volvo, Mack), International Trucks. Blink...Blink Blink.
      Getting a question or a suggestion for an article to me is as easy as
      Snail-mail Doug Rodgers
      P.O. Box 306
      405 Cedar Ave.
      Richland, NJ 08350-0306
      Keep Your Taillight Lit…Doug

Joe Laird
'78 Eagle
Sioux Falls, South Dakota

Offline akroyaleagle

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Re: How long should your air last?
« Reply #34 on: April 22, 2013, 03:19:22 PM »
I agree, self adjusters should not need adjusting. If they did, they wouldn't be "self adjusting".

They do require checking!

The travel of the actuator rod is the same as the manual adjusters. The checking is done the same way as the manual.

Alaskan and Western Canadian drivers stop, release the parking brake and apply the brakes a half dozen times or so.

That seems to ratchet them down correctly. (Remember the way to adjust auto adjusters on our cars is to back up and apply the brakes, or just hold a little pressure and back up.)

I do that occassionally and I have never found one of my slack adjusters out of adjustment. I have to remember to do it because I rarely apply brakes except just before a complete stop. My Jake does the rest. I see more and more trucks do the same.
I have found one that needed replacing due to age.
Joe Laird
'78 Eagle
Sioux Falls, South Dakota