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Rob

Is Is Possible For Manual Slack Adjusters To Strip Internally?

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This hit me kinda funny the other day. I had my daughter jump up into "Yella Dog" to apply and release the trolley valve so I could watch the brake hardware on the trailer actuate. I've never had hardly any brakes on this thing since the purchase. After ensuring the brakes were offering a slight drag on the drums, I had her pull the trolley valve and much to my surprise, all four chambers stroked, but only on cam physically turned and it was very little. I had her repeat this action several times yielding the same result each time. I then had here stand on the brake pedal and three of the slack actuated further, rotated the cams until three of them violently skipped internally allowing the brakes to fully release. The brakes were then extremly loose and could be adjusted back up via the adjusting bolts again. I then grabbed a pipe wrench and the cams rotate easily against the shoe retraction springs, and spread the brake shoes.

It's obvious I need new slack adjusters but I've never heard of something like this happening before. Anybody else seen similar? I'll take the old ones apart once they are replaced to see what makes them tick internally and maybe what went wrong?

Rob

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A lot of old slacks will back off if the slyde lock ring wears down and let's the adj bolt turn.or the ratchet is broken. I have noticed that the newer auto slacks seam to stay adjusted better than manual slacks. I still check them but stopping power is always greater with them. I think. However I still use manual slacks on trailers as some times I like the ease of adjusting them to suite the tractor that is pulling it. My tractor does not have good stopping feel to it.I feel the trailer is doing most of the work. More than it should be doing to stop..

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It is not uncommon at all to have a slack adjuster fail in this manner. It would be uncommon to find that three or more had failed at the same time.

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The common manual adjusted ones use a worm and spur setup...the adjuster screw has a worm gear attached. the spline part has an external spur gear that the worm drives. As the ends of the worm gear are fixed in postion by the slack adjuster housing, if the teeth are not broken or worn down so as to slip, I would think the the locking mechanism is shot or slipping...the ratio is such that the spur can "drive" the worm if the locking mechansim has failed...Tighten the brake down, then watch the adjustment nut as the brakes are applied, I think you will see it spin....

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I was wondering about this again. With the brakes dragging the drums and the adjustment bolts tightend firmly, (wheels will not turn with tractor pulling trailer) and the foot brake applied, all four snap over and past the adjustment point and the brakes all relax. These slacks have to be junk.

I've ordered all new slacks, chambers, clevises, hardware, and will send the shoes in for relining if the cores are good. The drums are virgin and show very little wear. They must have either been replaced with the last relining, or have not worked in many years as the trailer is a 1981 model.

Rob

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Go through the adjustment again. With the shoes as tight against the drum as you can adjust them, check the S cam to see how much ramp is left available. Then while watching, have someone step on the brake to see if it cams over.

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Go through the adjustment again. With the shoes as tight against the drum as you can adjust them, check the S cam to see how much ramp is left available. Then while watching, have someone step on the brake to see if it cams over.

Hi James, I did that prior as it was my first inclination and they are not camming over. I even used a 24" pipe wrench on the camshafts to try to cam them over and could not. I did pull the clevis pins and can manually, (with my hand) move the slack adjuster over an inch before it starts to try to rotate the camshaft(s). This is in the normal plane of operation for the chambers to stroke the slack adjusters.

I'll have all this stuff broken down within the next couple of days and will post photos. I'm going to bust the side plates off a slack adjuster to have a look see inside.

Rob

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Thats what I thought that they were rolling over. But with 'almost new' drums there would have to be no lining left at all and even then I don't think they'd roll over.

With the new chambers make sure to mount them in the correct holes based on the slack length. Don't rely on were they were mounted they could be wrong.

And there is a procedure to properly cut the push rod to length. The one I use is on MGM brakes website. I've done so many I ended up making a jig.

Never seize everything and lots of grease will keep them working forever.

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My profession from 1979 till 2000 was trailer maintenance... Saw lots of ungreased slack adjusters that were so sloppy in the worm drive that they'd use most of the booster stroke to barely offer any brake. On road trains running up through the centre of Australia back then in the wet season the cam bushes would bind up with fine bull dust mud and stick on, especially when fitted with BPW type slack adjuser return springs attached from the spare clevis holes to the booster bracket.

If the cam splines weren't worn, the inner sleeve of the adjuster would rust onto the cam and would quite often have to be gas axed off, even with the adjuster itself flogged out.

Also saw quite a few as previously mentioned where the adjuster lock ring was blasted to bits by rocks etc and would allow the brakes to unwind right infront of your eyes when doing a two man test in the workshop... Which reminds me of a common complaint of little or no brakes at all on the whole road train after driving through desert floods.

Turns out the bull dust mud would enter the booster plunger hole and almost set like concrete not allowing booster activation. Push rod boots were later offered as an accessory to try and eliminate this problem.

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My profession from 1979 till 2000 was trailer maintenance... Saw lots of ungreased slack adjusters that were so sloppy in the worm drive that they'd use most of the booster stroke to barely offer any brake. On road trains running up through the centre of Australia back then in the wet season the cam bushes would bind up with fine bull dust mud and stick on, especially when fitted with BPW type slack adjuser return springs attached from the spare clevis holes to the booster bracket.

If the cam splines weren't worn, the inner sleeve of the adjuster would rust onto the cam and would quite often have to be gas axed off, even with the adjuster itself flogged out.

Also saw quite a few as previously mentioned where the adjuster lock ring was blasted to bits by rocks etc and would allow the brakes to unwind right infront of your eyes when doing a two man test in the workshop... Which reminds me of a common complaint of little or no brakes at all on the whole road train after driving through desert floods.

Turns out the bull dust mud would enter the booster plunger hole and almost set like concrete not allowing booster activation. Push rod boots were later offered as an accessory to try and eliminate this problem.

Thanks for the info. I always enjoy trying to figure out what you guys from down under are saying.

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I would check if the cam shaft splines are good. Also did someone put on the wrong slacks with the wrong spline configuration. Does it have the long throw chambers on it , over taxing the slacks?? Good luck with it.

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This trailer had a combination of curved arm slack adjusters, and straight. Upon removal of the slacks from the trailer, all four of them were very loose in the worm drive(s). I purchased all new hardware except the cams which were not worn out for some reason. I had the original shoes relined with the exception of 1/2 of one wheel due to a bad core. It was therefore new. Also purchased new chambers to add spring brakes and an R12 valve, (I think) to control the brakes, (it did not originally have spring brakes). The drums were not bad but they were cut .015 to true them up. I also got new "Stemco" seals, and hubcaps as the axles use oil hubs. I did not replace the wheel bearings as they looked pretty good.

The trailer goes for sandblast next week to get it cleaned up. I've got some welding, and a new king pin plate to install in the neck, and then will replank it with white oak boards.

Thanks for all the help.

Rob

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Pictures?

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