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Online Stillwobbling

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Re: Sticking rear caliper
« Reply #10 on: January 06, 2019, 10:17:03 PM »
Scores in the bores shouldn't really matter. The piston seals on the seal not the walls of the bore. if you take the seal out and drop the piston in, it virtually rattles around in there. There's no metal on metal contact in a brake caliper. Naturally, I'd want to find out where the scoring had come from, but a good clean up as suggested should sort any lurking issues.

Offline DanceswithTiger

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Re: Sticking rear caliper
« Reply #11 on: January 06, 2019, 10:23:57 PM »
Good point. I suspect I have bad seals or pistons, not a problem with the bores. Will know more when I disassemble. Should have parts in a week or two. Given the likely forecast between now and then, I'll begin disassembly sooner.

What actually provides the force that retracts the pistons/pads? Is it a bias of the seals? Or is there another mechanism at work that I'm not thinking of?  I know mechanical disk brakes on a bicycle have small springs that help push the pads away from the rotor, but that's not the case here.


*Originally Posted by Stillwobbling [+]
Scores in the bores shouldn't really matter. The piston seals on the seal not the walls of the bore. if you take the seal out and drop the piston in, it virtually rattles around in there. There's no metal on metal contact in a brake caliper. Naturally, I'd want to find out where the scoring had come from, but a good clean up as suggested should sort any lurking issues.
Dave
Rochester, NY

'12 Tiger 12

Online Stillwobbling

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Re: Sticking rear caliper
« Reply #12 on: January 06, 2019, 11:02:38 PM »
Well as I said earlier in the thread, the retraction of the piston is just the relaxation of the fluid in the system when you let go of the lever or pedal. Brake fluid can't be compressed and as it's a sealed system once you back off the piston is sucked (pushed by barometric pressure) back into the caliper. But if the piston is jammed, the result is largely that additional fluid is pulled in from the reservoir meaning that the system gets kinda backed up and solid ie the piston can't actually retract any more.

Offline unsubtle

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Re: Sticking rear caliper
« Reply #13 on: January 07, 2019, 12:31:05 AM »
No, this is incorrect.  When you release the brake lever, the fluid reverts to atmospheric pressure. There is no suction. The piston in the master cylinder exposes a hole which connects the reservoir to the brake line specifically to bring the brake line to atmospheric pressure, sucking in any fluid needed to top up the enclosed volume of fluid as the pad wears.

The way it does work depends on the seals. You will have noticed that they have a square cross-section, not round like a normal O ring. For small movements of the piston, the piston does not slip through the seal. Instead the seal flexes so that the cross-section distorts from square to something more like a parallelogram. When the pressure is released, the seal springs back to its square configuration, pulling the piston back. It's a very small movement, but enough to stop the brake pad dragging on the disc.

If the piston moves more, either because the pad has worn or because the mechanic is taking up slack after pushing the pistons back, the pistons do slip through the seals. So the seals have three functions: stop the fluid leaking; act as a very short range spring to pull the pistons back; and allow the pistons to slip to take up wear in the pads.

Online CaptainTrips

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Re: Sticking rear caliper
« Reply #14 on: January 07, 2019, 01:23:49 AM »
*Originally Posted by unsubtle [+]
No, this is incorrect.  When you release the brake lever, the fluid reverts to atmospheric pressure. There is no suction. The piston in the master cylinder exposes a hole which connects the reservoir to the brake line specifically to bring the brake line to atmospheric pressure, sucking in any fluid needed to top up the enclosed volume of fluid as the pad wears.

The way it does work depends on the seals. You will have noticed that they have a square cross-section, not round like a normal O ring. For small movements of the piston, the piston does not slip through the seal. Instead the seal flexes so that the cross-section distorts from square to something more like a parallelogram. When the pressure is released, the seal springs back to its square configuration, pulling the piston back. It's a very small movement, but enough to stop the brake pad dragging on the disc.

If the piston moves more, either because the pad has worn or because the mechanic is taking up slack after pushing the pistons back, the pistons do slip through the seals. So the seals have three functions: stop the fluid leaking; act as a very short range spring to pull the pistons back; and allow the pistons to slip to take up wear in the pads.
:460: Thanks for posting this.
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Offline DanceswithTiger

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Re: Sticking rear caliper
« Reply #15 on: January 07, 2019, 02:29:40 AM »
Thanks for the clear explanation. Makes a lot of sense.

*Originally Posted by unsubtle [+]
No, this is incorrect.  When you release the brake lever, the fluid reverts to atmospheric pressure. There is no suction. The piston in the master cylinder exposes a hole which connects the reservoir to the brake line specifically to bring the brake line to atmospheric pressure, sucking in any fluid needed to top up the enclosed volume of fluid as the pad wears.

The way it does work depends on the seals. You will have noticed that they have a square cross-section, not round like a normal O ring. For small movements of the piston, the piston does not slip through the seal. Instead the seal flexes so that the cross-section distorts from square to something more like a parallelogram. When the pressure is released, the seal springs back to its square configuration, pulling the piston back. It's a very small movement, but enough to stop the brake pad dragging on the disc.

If the piston moves more, either because the pad has worn or because the mechanic is taking up slack after pushing the pistons back, the pistons do slip through the seals. So the seals have three functions: stop the fluid leaking; act as a very short range spring to pull the pistons back; and allow the pistons to slip to take up wear in the pads.
Dave
Rochester, NY

'12 Tiger 12

Offline Deep6blue

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Re: Sticking rear caliper
« Reply #16 on: January 07, 2019, 03:15:36 AM »
*Originally Posted by unsubtle [+]
No, this is incorrect.  When you release the brake lever, the fluid reverts to atmospheric pressure. There is no suction. The piston in the master cylinder exposes a hole which connects the reservoir to the brake line specifically to bring the brake line to atmospheric pressure, sucking in any fluid needed to top up the enclosed volume of fluid as the pad wears.

The way it does work depends on the seals. You will have noticed that they have a square cross-section, not round like a normal O ring. For small movements of the piston, the piston does not slip through the seal. Instead the seal flexes so that the cross-section distorts from square to something more like a parallelogram. When the pressure is released, the seal springs back to its square configuration, pulling the piston back. It's a very small movement, but enough to stop the brake pad dragging on the disc.

If the piston moves more, either because the pad has worn or because the mechanic is taking up slack after pushing the pistons back, the pistons do slip through the seals. So the seals have three functions: stop the fluid leaking; act as a very short range spring to pull the pistons back; and allow the pistons to slip to take up wear in the pads.

Wow, great explanation. Thank you


Online Icy

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Re: Sticking rear caliper
« Reply #17 on: January 07, 2019, 03:40:06 AM »
*Originally Posted by unsubtle [+]
No, this is incorrect.  When you release the brake lever, the fluid reverts to atmospheric pressure. There is no suction. The piston in the master cylinder exposes a hole which connects the reservoir to the brake line specifically to bring the brake line to atmospheric pressure, sucking in any fluid needed to top up the enclosed volume of fluid as the pad wears.

The way it does work depends on the seals. You will have noticed that they have a square cross-section, not round like a normal O ring. For small movements of the piston, the piston does not slip through the seal. Instead the seal flexes so that the cross-section distorts from square to something more like a parallelogram. When the pressure is released, the seal springs back to its square configuration, pulling the piston back. It's a very small movement, but enough to stop the brake pad dragging on the disc.

If the piston moves more, either because the pad has worn or because the mechanic is taking up slack after pushing the pistons back, the pistons do slip through the seals. So the seals have three functions: stop the fluid leaking; act as a very short range spring to pull the pistons back; and allow the pistons to slip to take up wear in the pads.

Respect!  :460: :821:
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Online Stillwobbling

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Re: Sticking rear caliper
« Reply #18 on: January 07, 2019, 08:36:31 AM »
I'm sorry but where was the part that I mentioned that it was barometric pressure that was at play here - incorrect? There are a number of forces that retract the piston including the seals etc, but what I was attempting to explain was it's not necessary to add a spring to force the pads apart. However, if you have a sticking piston, the forces in the system are so small that they are insufficient to retract the piston. If it's the pad that's sticking, then the piston can't pull the pad back in any case. I know how the system works as I have a MC on the bench at the moment having replaced one where the seals had gone, but forgive me for not being quite so eloquent in describing its function.  I was trying to say in simple terms press the pedal, the piston extends, release the pedal and the piston retracts. If the piston doesn't retract then the pads are in constant contact with the rotor which will lead to accelerated wear and drag.

The answer is not to add or modify the system, but simply service it to get it back to operating condition.

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Re: Sticking rear caliper
« Reply #19 on: January 07, 2019, 11:46:47 AM »
*Originally Posted by Stillwobbling [+]
I'm sorry but where was the part that I mentioned that it was barometric pressure that was at play here - incorrect?

That would be in post # 12....

 


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