Author Topic: Rear Servo Brake Over-Adjusting!  (Read 655 times)

Offline ehjorten

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Rear Servo Brake Over-Adjusting!
« on: June 03, 2019, 09:23:30 AM »
Tore into the rear brakes on my Crew Cab.  I had redone the brakes, but before the LS swap the Driver's side Rear was dragging heavily and would get hot!  I would dial back the adjustment and it was just like it was over-adjusting!  I couldn't understand why it would keep adjusting tighter and tighter.  I have never had that happen before.  The only thing that I could find was a little bit of weeping brake fluid on the forward cup of the wheel cylinder.  So...I replaced the wheel cylinder.  I had replaced it when I did the brakes about 30k miles ago.

The secondary shoe was cracked in two places across the rivets and there was some missing lining material.  I can only guess that that happened because of the heat and was not a cause.



Brakes Redone!



My guess is that the leaking on the primary shoe side of the wheel cylinder was causing some uneven application of the brake shoes, that was activating the adjusting lever and slowly ratcheting the star-wheel more than it should have been.  I have never had something like this happen before.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline JohnnyPopper

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Re: Rear Servo Brake Over-Adjusting!
« Reply #1 on: June 04, 2019, 06:37:00 AM »
Wow, that is really bizarre.

Normally the star is only moved as the result of reverse torsion on the entire assembly.

But I get your theory; maybe the shoe would grab and release, causing the assembly to oscillate between forward and reverse torsion.

Not a usual suspect, but I would have the drum checked out too.
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Offline ehjorten

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Re: Rear Servo Brake Over-Adjusting!
« Reply #2 on: June 04, 2019, 09:01:20 AM »
Drum and bearings were brand new when I last did the brakes about 30k miles ago.  Drum was pressed-on, new bearings and races inserted, and a light clean-up pass was taken on the drum to true up the braking surface!  The drag from the shoes has always been nice and even.

The drum brakes that we have on our vehicles (Duo-Servo Drum Brakes) automatically adjust when the brake linings wear down to a certain point that when you actuate the brakes and the shoes come off of the top anchor pin, they rotate enough from the spinning action of the drum and the friction between the linings and drum.  I am thinking that if one side was not being pushed off of the anchor the same as the other, then it would look like there was more distance between the linings at their resting point...which would activate the star wheel.  That is all that I could find in there was that brake fluid that was leaking from one side of the wheel cylinder.  Prior to re-doing the brakes everything worked fine, and the passenger's side still does work fine.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline bd

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Re: Rear Servo Brake Over-Adjusting!
« Reply #3 on: June 04, 2019, 09:27:08 AM »
Was there enough fluid leakage to actually reach the shoe?  Because of the rotation of the secondary shoe away from the anchor pin (CW on the driver side; CCW on the passenger side), which more effectively tugs on the adjuster levers, self-adjusters are more apt to tighten when backing than when moving forward.  Do you consistently turn the steer wheels in the same direction while backing and then braking, for example, out of your driveway or parking spot at work, etc?  This can result in differential over-adjusting of one rear brake, left vs right.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bd

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Re: Rear Servo Brake Over-Adjusting!
« Reply #4 on: June 04, 2019, 09:30:26 AM »


Incidentally, this type of friction material failure can also result from improper curing during manufacture.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)