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Caliper O-ring part number

Matthew P
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N204TA Enthusiast

N204TA

Posted
Posted (edited)

The IPC shows an AN6227B-29 / MS28775-326 o-ring.

How are your brake pads?  I’d suspect worn pads causing the caliper piston to leak before a problem with the o-ring.

Edited by N204TA
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Shadrach Grand Master

Shadrach

Posted
Posted
  On 8/25/2024 at 11:16 PM, N204TA said:

The IPC shows an AN6227B-29 / MS28775-326 o-ring.

How are your brake pads?  I’d suspect worn pads causing the caliper cylinder to leak before a problem with the o-ring.

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How would a worn pad cause a caliper to leak?

N204TA Enthusiast

N204TA

Posted
Posted
  On 8/26/2024 at 12:35 AM, Shadrach said:

How would a worn pad cause a caliper to leak?

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Worn pads require the piston to move further when applying the brakes.  Eventually, it will be sticking out far enough that hydraulic fluid can leak around the o-ring.

Shadrach Grand Master

Shadrach

Posted
Posted
  On 8/26/2024 at 12:58 AM, N204TA said:

Worn pads require the piston to move further when applying the brakes.  Eventually, it will be sticking out far enough that hydraulic fluid can leak around the o-ring.

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IIRC the cylinder wall is the same dimension from top to bottom with perhaps a slight chamfer at the top. It should not leak as long as the o-ring is in contact with the cylinder wall. I cannot imagine a scenario where that would happen even with very worn pads. Is this something you’ve seen a lot of?
I rebuilt my calipers years ago because of an o-ring failure. They do fail but it is rare.

MikeOH Grand Master

MikeOH

Posted
Posted
  On 8/26/2024 at 1:19 AM, Shadrach said:

IIRC the cylinder wall is the same dimension from top to bottom with perhaps a slight chamfer at the top. It should not leak as long as the o-ring is in contact with the cylinder wall. I cannot imagine a scenario where that would happen even with very worn pads. Is this something you’ve seen a lot of?
I rebuilt my calipers years ago because of an o-ring failure. They do fail but it is rare.

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Do we need a FOIA request to get that redaction removed?:D

  • Haha 1
EricJ Grand Master

EricJ

Posted
Posted
  On 8/26/2024 at 1:19 AM, Shadrach said:

IIRC the cylinder wall is the same dimension from top to bottom with perhaps a slight chamfer at the top. It should not leak as long as the o-ring is in contact with the cylinder wall. I cannot imagine a scenario where that would happen even with very worn pads. Is this something you’ve seen a lot of?
I rebuilt my calipers years ago because of an o-ring failure. They do fail but it is rare.

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Mine did that the last time the pads got worn down, i.e., first symptom is a little wetness around the bottom edge of the pad.   These sorts of little oozy leaks seem to be a little worse with the synthetic fluid, but I've yet to see one turn into an actual drippy leak.

 

MB65E Grand Master

MB65E

Posted
Posted
  On 8/25/2024 at 11:16 PM, N204TA said:

The IPC shows an AN6227B-29 / MS28775-326 o-ring.

How are your brake pads?  I’d suspect worn pads causing the caliper piston to leak before a problem with the o-ring.

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I’ll confirm the -326 oring should be correct. Used this on my 65E a few years ago. Had the same issue confirming. 
-Matt

47U Mentor

47U

Posted
Posted
  On 8/26/2024 at 1:50 AM, EricJ said:

Mine did that the last time the pads got worn down, i.e., first symptom is a little wetness around the bottom edge of the pad.   These sorts of little oozy leaks seem to be a little worse with the synthetic fluid, but I've yet to see one turn into an actual drippy leak.

 

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The brake calipers in my Cherokee (circa 1998) were so neglected that the cylinders were corroded on the outer surface, past where the o-ring rides.  The wetted area of the cylinder was fine.  But, when the pads were worn down to the rivets the o-ring was now trying to seal against the rough cylinder wall surface and the leak ensued.  I was able to clean up the surface with emory cloth and scotchbrite.  And new break disks that didn’t wear the pads out every year.

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Shadrach Grand Master

Shadrach

Posted
Posted
  On 8/26/2024 at 4:57 AM, 47U said:

The brake calipers in my Cherokee (circa 1998) were so neglected that the cylinders were corroded on the outer surface, past where the o-ring rides.  The wetted area of the cylinder was fine.  But, when the pads were worn down to the rivets the o-ring was now trying to seal against the rough cylinder wall surface and the leak ensued.  I was able to clean up the surface with emory cloth and scotchbrite.  And new break disks that didn’t wear the pads out every year.

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When I overhauled my calipers, you could see some light, surface corrosion on the unwetted area but mine never leaked. It may have contributed to the o-rings demise, but it had been in service for forty years so it owed us nothing. I lightly honed the cylinder walls and replaced the o-rings It’s been good for 17 years.

Shadrach Grand Master

Shadrach

Posted
Posted
  On 8/26/2024 at 5:25 PM, Matthew P said:

Actually, I'm not boring cylinders I'm bleeding brakes and changing o-ring on caliper

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The o-ring fits in a groove machined into the caliper piston. The piston will need to be removed to replace it.
IMG_0468.jpeg.2da6fcbfd6e772382adc3a463aa3a713.jpeg

It would be a shame to go through the trouble to remove everything and not have the tools to repair.  Your IA likely has a plan for such contingencies.

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