Brake chamber push rod length

Discussion in 'Trucks [ Eighteen Wheelers ]' started by 440CHEVY, Feb 5, 2009.

  1. 440CHEVY

    440CHEVY Bobtail Member

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    Jan 13, 2009
    Duluth, MN
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    First of all i'm very new to all this, and I will be bringing it to someone to have them adjust the brakes and check over everything. Anyway, I bought a used end dump and both of the brake chambers on the right side were really nasty and rusted out, so I grabbed some new ones today and put them on but i'm a little unsure on what the push rod length needs to be. The only one I ended up doing was on the front axle, and the old push rod measured at 6" (when it was off the truck) but the one still on the truck measured at 8.5", so I ended up cutting it at 8.5" after taking the caging bolt out. Does this sound right? I figured if I was off I could always cut off more. But when I installed it on the trailer the slack adjuster's didn't line up with each other at all, and the one I didn't touch was out further then my new one. So, that right there got me worried that I had just ruined 70 bucks of brake chamber. I was thinking that I might need to just replace both the brake chambers on the front axle (and if so just replace them all) but, I really have no clue. Thanks for any help.
     
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  3. 440CHEVY

    440CHEVY Bobtail Member

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    Jan 13, 2009
    Duluth, MN
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    Oh, and if it matters it's a 2003 Trailstar 24' trailer.
     
  4. emtepockets

    emtepockets Light Load Member

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    Jan 12, 2009
    Poplar Bluff, MO
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    no it dosent matter on that, the differance you are seeing may be coming from the condition of the brake shoes themselves.
    one side may be thinner than the other one. usually when the brakes are released, wheels chocked of course, the push rod travel should be about one inch. that is with the brakes properly adjusted. i keep my brakes adjusted to this, 1/2 turn on rear tandems and drivers, and 3/4 turn on the steers.
     
  5. heyns57

    heyns57 Road Train Member

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    Dec 30, 2006
    near Kalamazoo Speedway
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    When the brake is applied, the angle between the rod and the slack adjuster should be 90 degrees. The wrong rod length can put you out of service in an inspection. Could you have taken an old chamber to the supplier, or compared part numbers?
     
  6. 440CHEVY

    440CHEVY Bobtail Member

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    Jan 13, 2009
    Duluth, MN
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    I did take the old chamber but they just gave me a similar one, didnt match any numbers. I guess I may have to just replace all the chambers, but I'm just trying to figure out how to set the push rod length. Is there a good rule of thumb for where I should set it at first? Oh and all of the brake shoes were new when I bought it.
     
  7. heyns57

    heyns57 Road Train Member

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    Dec 30, 2006
    near Kalamazoo Speedway
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    Brake chambers are available with different lengths of stroke. From your info, you had a 2.5" stroke with the original chambers. That is about standard. Install the canister with the spring brake caged. The shoes are not applied. The 6" of brake rod showing should touch the slack adjuster which is at the beginning of it's arc. Half way through the arc, the angle of the rod to the adjuster should be 90 degrees. With the spring brake cage removed, the 8.5" of rod should touch the adjuster after you pull the adjuster manually to the applied position.

    Holes in the slack adjuster allow you to change the length of the push rod arc, but I have not seen many rods attached anywhere but the longest hole. If you change holes in the slack adjuster, you are changing brake input. These things have been engineered as a system. It is possible to have too much input making the brakes oversensitive. If you increase the length of the slack adjuster, you have more leverage but the brake chamber may "bottom out". Also, the brake rod has to be proper length to prevent binding of the push rod in the chamber. Watch the angle at which the push rod enters the chamber.

    These ideas came from Owner Operator magazine Mar/Apr 1973, page 98.
     
  8. Bigstretch

    Bigstretch Light Load Member

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    Jun 14, 2008
    NW IL
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    The best way to cut the rods to the proper length, you apply shop air to the "service" port on the old chamber, lock in place with Vise Grips and measure the rod length. Do the exact same with the new one. Mark the correct length on the rod and cut accordingly. Put the clevis in the same position as it was on the old one.

    Put anti-seize compound on ALL of the hardware threads, and put lube on the hose swivels to make replacement easier later on. Also prefer thread dope over tape on the fitting threads, for the same reason.

    On the adjustment, it depends. Typically between 1/4 and 1/2 of a turn back from full contact with the drum. On some, a half turn back is too much (spring brakes won't hold on hills). I do 1/3 back on all. Works well, and not too tight, unless you are running a tri-axle trailer that isn't used very often.

    There are valves that regulate the pressure and timing from front to rear, so I set them adjustment the same on all axles. Done it that way for years, and no problems. If you have auto-slack adjusters, make sure they are working properly. We have some on one trailer, and I have to manually adjust them. It is due for new brake shoes and they'll be getting replaced.

    If you run locally, with lots of loads and/or frequent stops, and manual adjust slacks, I readjust them for wear on a weekly basis, and grease the whole truck weekly. May seem excessive, but I've never had to replace king pins or ball joints in over 10 yrs (and we have some really chitty roads around here, too). Still good as new.

    My boss likes the cheap crap grease (no EP additives), but I buy the better stuff that has the EP additives and high temp. VERY tacky, and lasts longer. Also, being that it is red and not brown/black in color, I can tell easily when the dirty grease is coming out.
     
  9. 440CHEVY

    440CHEVY Bobtail Member

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    Jan 13, 2009
    Duluth, MN
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    Thanks for the posts guys, very good info. I ended up just buying all new slacks and chambers and so far I have the rear axle done. It does have auto-slack adjusters, and I measured the two push rods off the rear axle at right around 7.75. So, I just cut them at 8 and then checked them installed with the specs on the new slack adjusters and they were right in the middle of what the directions said. Tested it out and everything seemed to work well, but I notcied that on one side the brake shoe was very close to the drum, whereas the other side the gap was more noticable. Is that something to worry about? Also as it stands when the brake is applied they dont quite go all the way to 90 degrees. Other then that though just replacing the parts on those two brakes I noticed the trailer brakes were working much better.

    What a PITA to replace those slack adjusters though...under there with a torch and a hammer for way to long. But, the knowledge from doing this is definetly worth it.
     
  10. AgLaw

    AgLaw Light Load Member

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    Jun 27, 2008
    Wharton, Texas
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    By now you've likely completed your repairs. If all is working well, pat yourself on the back because I've had chambers replaced on a Freightliner at the local dealer, and they just slapped it all together without double checking pushrod length and clearances. The truck made it about 200 miles when the wheel on which the chamber was replaced caught fire--it was over adjusting. Luckily a few fellow truck drivers on IH10 between Houston and San Antonio stopped to help put the fire out. We had to replace one tire, but everything else was fine.

    Those were Rockwell/Meritor's stroke-sensing (w/ lift/actuator rod) type automatic slack adjuster. Personally, I hate them as they are very unforgiving. Everthing must be within very close tolerances for them to work and operate properly. Too much, they over adjust...too little, they under adjust.

    The clearance-sensing (have an external bolt- or clamp-on bracket) automatics by Haldex and Meritor are much more forgiving. In addition you don't need a special clevis to accomodate the actuator/lift rod on the slack adjuster.

    Regarding the 90 degree on application...sounds like you've got them correct. They should only be "approaching" a right angle upon application as a right angle transfers the maximum torque offered by the chamber. If you are at or over 90 degrees under normal conditions, then upon any fade, you will be losing available torque as the stroke progresses. You want that 90 degrees to show up during severe braking--thus you'll be gaining the available torque from your brake chamber. However, always follow the instructions provided by the manufacturer.

    Good luck.
     
  11. kd5drx

    kd5drx <strong>Master of Electronic Communications</stron

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    Nov 28, 2006
    Some where USA
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    The gap between the shoe and the drum is where the slack adjuster comes in you need to do an initial adjustment manually then if you put the auto adjusters on they should maintain the adjustment. Manually adjust them like has been suggested 1/2 to 1/4 turn back off of full contact that will get you in the right spot.
     
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