How do self-adjusting brakes work?

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whc03grady
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How do self-adjusting brakes work?

Post by whc03grady » Thu Mar 25, 2010 8:39 pm

Maybe this is unrelated to the title. I put new pads on our 1971 Squareback. With the old pads out, the pistons in the caliper press together, to the point where you need to press them back in to fit the pads.
The question is, why doesn't this happen all the time? I assume it has to do with that funny-shaped spring, the cross thing, but if so, what? Is this all related to the self-adjusting property of the brakes?
Ludwig--1974 Westfalia, 2.0L (GD035193), Solex 34PDSIT-2/3 carburetors.
Gertie--1971 Squareback, 1600cc with Bosch D-Jetronic fuel injection from a '72 (E brain).
Read about their adventures:
http://www.ludwigandgertie.blogspot.com

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sgkent
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Post by sgkent » Fri Mar 26, 2010 11:52 pm

that is a good question.

Generally disk brakes are not considered "self-adjusting" like rear brakes that adjust through the star wheel when you back up. Disk brakes might be considered more self-canceling than self-adjusting. When they are working right, the minute pressure is released, vibration and runout in the disk pushes them back into the caliper until they are just clear of the disk. There isn't anything that would push them all the way back into the caliper bore except you or a tool. Some brakes have a spring that dampens the pads. On the VW ones the spring is that cross thing you spoke of and it would apply a small amount of pressure to push the pads back from the disk. Most disk brakes don't have that same style spring.
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Amskeptic
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Re: How do self-adjusting brakes work?

Post by Amskeptic » Sat Mar 27, 2010 8:27 am

whc03grady wrote:Maybe this is unrelated to the title. I put new pads on our 1971 Squareback. With the old pads out, the pistons in the caliper press together
Residual circuit pressure. If you opened a bleeder screw (don't!) it would not happen. Same occurs with drum brakes when the shoes/springs are off.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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whc03grady
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Re: How do self-adjusting brakes work?

Post by whc03grady » Sat Mar 27, 2010 9:38 am

Amskeptic wrote:If you opened a bleeder screw (don't!)....
(Uh-oh.)
I don't own a brake cylinder compressor, so to push the pistons back in in order to install the pads, I pressed them in with a blunt wooden instrument as I opened the bleeder screw with my other hand. I would tighten the screw back in as fluid was coming out, the thought being no air would enter the system if I did it that way. Wrong-o?
Ludwig--1974 Westfalia, 2.0L (GD035193), Solex 34PDSIT-2/3 carburetors.
Gertie--1971 Squareback, 1600cc with Bosch D-Jetronic fuel injection from a '72 (E brain).
Read about their adventures:
http://www.ludwigandgertie.blogspot.com

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whc03grady
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Re: How do self-adjusting brakes work?

Post by whc03grady » Sat Mar 27, 2010 9:40 am

Amskeptic wrote:Residual circuit pressure.
But wait! Why doesn't this same residual circuit pressure act to keep the calipers/shoes under, uh, pressure all the time (and thus why aren't the brakes "on" all the time)?
Ludwig--1974 Westfalia, 2.0L (GD035193), Solex 34PDSIT-2/3 carburetors.
Gertie--1971 Squareback, 1600cc with Bosch D-Jetronic fuel injection from a '72 (E brain).
Read about their adventures:
http://www.ludwigandgertie.blogspot.com

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covelo
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Re: How do self-adjusting brakes work?

Post by covelo » Sat Mar 27, 2010 10:41 am

whc03grady wrote:
Amskeptic wrote:If you opened a bleeder screw (don't!)....
(Uh-oh.)
I don't own a brake cylinder compressor, so to push the pistons back in in order to install the pads, I pressed them in with a blunt wooden instrument as I opened the bleeder screw with my other hand. I would tighten the screw back in as fluid was coming out, the thought being no air would enter the system if I did it that way. Wrong-o?
No need to open the bleeder screw. We used a nice flat crowbar to gently persuade the pistons back into their bores when we replaced the disk brake pads on our bus last summer. You just have to remember to remove the excess brake fluid with a clean syringe before it bubbles out of your reservoir and all over your front seat area.
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whc03grady
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Re: How do self-adjusting brakes work?

Post by whc03grady » Sat Mar 27, 2010 10:46 am

covelo wrote:No need to open the bleeder screw. We used a nice flat crowbar to gently persuade the pistons back into their bores when we replaced the disk brake pads on our bus last summer.
It didn't feel like there was any give without loosening the bleeder screw. I'll definitely try harder when I do the other side (why didn't I do them at the same time? Long story...) next week.
covelo wrote:You just have to remember to remove the excess brake fluid with a clean syringe before it bubbles out of your reservoir and all over your front trunk area.
(Fixed it--It's a Type III, but thanks for the heads up.)
Ludwig--1974 Westfalia, 2.0L (GD035193), Solex 34PDSIT-2/3 carburetors.
Gertie--1971 Squareback, 1600cc with Bosch D-Jetronic fuel injection from a '72 (E brain).
Read about their adventures:
http://www.ludwigandgertie.blogspot.com

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Amskeptic
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Re: How do self-adjusting brakes work?

Post by Amskeptic » Sat Mar 27, 2010 7:10 pm

whc03grady wrote:
covelo wrote:No need to open the bleeder screw. We used a nice flat crowbar to gently persuade the pistons back into their bores when we replaced the disk brake pads on our bus last summer.
It didn't feel like there was any give without loosening the bleeder screw. I'll definitely try harder when I do the other side (why didn't I do them at the same time? Long story...) next week.
covelo wrote:You just have to remember to remove the excess brake fluid with a clean syringe before it bubbles out of your reservoir and all over your front trunk area.
(Fixed it--It's a Type III, but thanks for the heads up.)
If opening the bleeder screw helped you to retract the pistons, I do not have a problem with that because the only rule I am trying to honor here is "never open the bleeder screw without positive pressure".

The system is designed to open the reservoir to the lines when your foot is off the brake pedal. This allows heat-induced expansion of the fluid to go back up to the reservoir, and it also allows the fluid to fill in the cylinders as pads/shoes wear. Sometimes, dragging brakes are caused by blocked compensating ports in the master cylinder due to filthy fluid and sludge. People will think it is the brake hose, when in fact, the bleeding process after the hose is replaced is what clears the ports!
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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