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Old 12-19-2014, 12:19 PM   #1
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"Centering" Brake Backing Plate

I'm still struggling with my left rear trailer brake grabbing at slow speeds. I read on another forum that a potential cause is the brake backing plate being off center, and a centering procedure was identified.

My question is, is this a potential problem with the 2001 vintage dual axle trailers? If a PO hit a pothole, ran over a curb, etc. is it possible that the brake backing plate on the rear axle might have gotten knocked off center?

Another possibility I'm pursuing is I may have the boost set too high (B1). My trailer weighs around 6000# and the GVWR of the TV (which I'm probably close to) is 7200#. Per the P3 manual B1 is the maximum setting I should be using. I would have thought that if the boost was too high, more than one axle would be grabbing.

Thanks for any suggestions.

Al
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Old 12-19-2014, 01:55 PM   #2
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As a ramdom thought related to cars. The brake shoes in most cars and light trucks are not the same length. The smaller shoe is towards the front of the backing plate and the larger one is placed on the rear. Mix it up and one issue is a grabbing brake.

I throw this out as a possibility and not necessarily as a diagonsis. Trailer shoes may be the same length.

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Old 12-19-2014, 02:31 PM   #3
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Yes the smaller shoe is the forward shoe.

As for centering. I have seen some backer plates with slightly larger holes than are necessary for the bolts. What I have done in that case is machine 2 bushings that fit over the blot and fill the holes in the backer plate. Position the plate using those 2 bolts and bolt down the other 3 before removing the bushings. If all 5 bolts are tight that should hold the plate on center.

Another thing that will cause grabbing is if the trailer is not parallel to the ground. The higher axle will have a tendency to grab because of reduced weight on it.

Most controllers suggest traveling at 25 mph on a gravel road and fully activating the manual control arm. If the brakes lock back off and repeat the test till the breaks just stop locking. That is max. braking.
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Old 12-19-2014, 02:32 PM   #4
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Action: Thanks for the suggestion. As I recently re-packed the wheel bearings and cleaned, lubricated, and adjusted the brakes I am reasonably certain (but not 100%) that the shoes are in correctly. On the Airstream, the small shoe goes in front, if I remember correctly. Mine should all be the same, but only one brake is grabbing.

All: I was meticulous about cleaning the drums after repacking, so I doubt that grease contamination is the problem unless a grease seal failed.

I have owned the trailer for about a year, but didn't use it seriously until this summer. I had to add a brake controller to my truck. When I first towed the trailer I followed the instructions for setting up the controller and advanced the gain until I heard the brakes lock up when fully applied with the panic lever. I backed off on the controller gain and all appeared fine. About 1000 miles into a 1500 mile trip the grabbing at low speed started. It's possible that what I heard initially was only one wheel locking up.

Also, the wheel is locking up with only very light pressure on the pedal. I would think that if it were due to tilt or loading it wouldn;t happen so early in the pedal travel. I could understand one wheel locking up before the others, but this happens with the initial, very light, application of the brakes.

Al
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Old 12-19-2014, 03:11 PM   #5
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Does it run hot when the brakes are not being used? My suspect is grease on the drum or some really bad mechanical issue. I had one wheel run hot and it blew the seal out and the grease caused it to lock easily. This is one that ran hot because it was adjusted too tight and/or it was not broken in yet. Make sure the magnet its properly attached to the lever and that the spring behind the magnet is working. A little drag on that magnet will cause it to stay on. Old worn out magnets can cause problems or using old magnets on a new drum.

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Old 12-19-2014, 03:34 PM   #6
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The other thought I have is brake adjustment. That wheel being different that the other wheels.

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Old 12-19-2014, 03:44 PM   #7
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I would back the adjustment screws off that wheel and see if it makes a difference.

I have the same situation in that if a wheel is to lock up it is always the same drivers rear wheel. This still happens even after a full brake job. New backing plates, drum, shoes, magnet.

The trailer seems to brake just fine otherwise, just after a good amount of driving and things are hot, that wheel gets touchy.
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Old 12-19-2014, 04:01 PM   #8
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Thanks Everyone!

The brake does not drag or lock. I check hub temps each time I stop and they are all always only slightly warm. A response to my original thread suggested adjustment so I readjusted all 4 brakes, being careful to get them with the same drag as best I could judge - no change.

In the interest of full disclosure, this wheel had a worn magnet and a scored magnet surface when I serviced the brakes in September. I replaced the magnets and turned the magnet surfaces on both rear axle drums. Prior to that service, I had not noticed a problem. Maybe the PO had the adjustment backed off on that drum. I know its usually conclusive that the last time it was touched is what created the problem, so I'm pulling the trailer from storage tomorrow to pull the drum and inspect again. If I find anything, I'll post it here.

Timmah: Enjoy your blog!

Al
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Old 12-19-2014, 04:05 PM   #9
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Your quote .........
Quote:
Originally Posted by Al and Missy View Post
Thanks Everyone!

..... The brake does not drag or lock. .....
Al
You said above,
"I'm still struggling with my left rear trailer brake grabbing at slow speeds."
"Also, the wheel is locking up with only very light pressure on the pedal."

So now I am confused.

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Old 12-19-2014, 04:15 PM   #10
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I had this problem on my 27' Overlander. After exhausting all other possibilities I had the drum turned. Problem solved, It looked just fine but the drum must not have been perfectly round.
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Old 12-19-2014, 08:34 PM   #11
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Action:
Sorry for the mixed terminology. Instead of "Also, the wheel is locking up with only very light pressure on the pedal.", I should have said "Also, the wheel is grabbing with only very light pressure on the pedal."

What I meant was the brake was not dragging or locking without pedal pressure, but when even slight pedal pressure is applied the brake goes to full on and the wheel locks up.

Sorry for the imprecision.

Al
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Old 12-19-2014, 09:08 PM   #12
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Quote:
Originally Posted by Al and Missy View Post
What I meant was the brake was not dragging or locking without pedal pressure, but when even slight pedal pressure is applied the brake goes to full on and the wheel locks up.
Pretty certain that is how we all understood it from the beginning.

Why not try adjusting that single wheel back off more than the others?

Thanks for the kudos on the blog. I can't take much credit as it is mostly the wife.
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Old 12-19-2014, 09:14 PM   #13
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Throwing out another thought ...... is it possible the brake magnet on that one wheel is different than the others?

Newer versus older
Round verus oval
Brake wiring different than the other 3 wheel such that the current or voltage is different?

Just grasping at anything.

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Old 12-19-2014, 09:14 PM   #14
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Also per your boost on your P3 thought:

Quote:
Another possibility I'm pursuing is I may have the boost set too high (B1)
I'd say you are good using B1. I leave mine on B1 80% of the time and up it to B2 on interstates or when descending mountains.

The boost is just how fast the braking power is applied, not the max level of power. You might want to try and tweak the power level voltage (forget what it is called) and see what that does. (Assuming you haven't messed with it much) I know for me there is a certain perfect sweet spot. Few tenths too high the trailer gets really grabby and a few tenths lower and I feel I want more trailer braking.
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Old 12-19-2014, 09:23 PM   #15
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Timmah:
As I understand it, the boost determines how much current is supplied in the first five seconds. At B0, the current rises from 0 to the level predicted by the deceleration sensor. B1 gives an initial pulse at 15% more than demand, B2 and B3 are progressively higher. When I pick up the trailer tomorrow I'm going to test it at B1 and if it is still grabbing I'm going to try B0. My trailer weighs about 1000# less than my truck. The manual says I should be at B0 or B1.

Action:
The only way that magnet is different is if it is defective. It matches the other three in appearance. It was ordered as the magnet for 12" Reliable brakes, and both magnets on the rear axle were replaced at the same time.

Al
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Old 12-19-2014, 09:30 PM   #16
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I'm saying regardless of the boost you may have too much voltage going to your brakes than what is needed for smooth operation. If you are in testing mode tomorrow try dialing back the voltage as well. See if that makes a difference.
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Old 12-20-2014, 08:00 AM   #17
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I would jack up the trailer so you can spin the wheels.

Spin each wheel to test for smooth rotation and no high spots in the drums. If the wheel in question is the odd one the drum may be warped or a return spring may have broken. If all is OK have someone is the TV. Spin a wheel and call for them to manually apply the brakes. The wheel should slam to a stop. Repeat with each wheel.

If only the wheel in question stops the wiring is the problem.

If the test don' show the problem I would remove the drum that is locking and do a visual inspection.
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Old 12-20-2014, 09:06 AM   #18
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Timmah:
I didn't mention it before, but my first reaction was to turn down the current. It was originally set between 8 and 9 and for the first 1000 miles that was great. On the last trip back to storage I was testing it and turned it down to 6 with no improvement.

HowieE:
I do remember that one or more of the brakes had a very slight high spot I noticed when I was adjusting the brakes. How much, if any, of a high spot is acceptable? With a wheel spinning, I could hear a swish-swish as the wheel(s?) went around.

Maybe I should just go for it and have the shoe surface of the drum turned. There is plenty of thickness left or just bite the bullet and replace the drums. It probably wouldn't be much more expensive and about the same amount of work (got to pull the bearings anyway).


Thanks Guys!

Al
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Old 12-20-2014, 09:49 AM   #19
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Hi, remove that one wheel and check the brake shoe return springs. A weak spring can cause this too. If not a broken, or obviously weakened spring, [gaps between coils or burned off paint] do the drop test. A good spring will have a little twang to it and a bad spring will just thud. Drop the springs on a concrete driveway or garage floor.
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Old 12-20-2014, 01:07 PM   #20
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Picked up the trailer and drove home. After warming up the brakes, turned the boost off and adjusted the current. Just starting to lock up at 8.2 or 8.3 but I can't tell how many brakes are locking. Gonna have to go to a parking lot or something. I really prefer the braking action at B1, but I could live with B0. Drove all the way home without any grabbing at B1 and current 8.0. I'm going to pull the wheel anyway. While I have it off the ground I'll check the braking action on both brakes, but the current looks right for all 4 magnets working. I'll also pull and check the springs on that wheel.

Thanks for all the suggestions.

Al
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