Be careful with factory calipers

Vetracr

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Mar 16, 2018
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I installed BMW 335i calipers with 380x30mm drilled and slotted rotors and Cool Carbon ST brake pads on my E89 Z4. After the last outing I went to get gas. Pulling back into garage I had low brake fluid light come on. I found left front caliper leaking fluid. Note I had not worn brake pads enough to activate wear indicator light and caliper dust boots looked fine. They weren't cooked or cracked. Needless to say I thanked my lucky stars it hadn't happened at speed the night before. To be safe I replaced both front calipers along with new brake pads and bled the brakes. I changed rear pads as long as I was doing front. Note both calipers had been on the car for maybe 3000 miles. After finishing the repair I rebuilt calipers I took off car for spares. I was shocked when I pulled the caliper seals. Both the left and right seals had pieces of rubber missing as if they had been dug out. The left seal was leaking but the right seal was ready to let go as well. The pistons looked fine with no blemishes to damage seals. See attached photos. Has anyone seen a failure like this before and if so what is cause of this failure mode. I've never seen anything like it.

Larry
Passenger side seal ready to go.
20180905_105240-001.jpg

Driver side seal leaking
20180905_104641-001.jpg
 
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NoQuarter

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Nov 24, 2017
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That is alarming.

Doesn't even look heat related but I suppose that is the only thing that makes sense to affect both sides the same.
 

silverstreak18

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Jan 3, 2018
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Almost looks like chunks taken out with a screwdriver. You said the calipers were on there for 8 months, were they new or bought used? Maybe someone had the calipers off, stepped on the brake pedal and popped out the pistons. Then did a hack job of putting them back in?
 

Vetracr

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Mar 16, 2018
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Almost looks like chunks taken out with a screwdriver. You said the calipers were on there for 8 months, were they new or bought used? Maybe someone had the calipers off, stepped on the brake pedal and popped out the pistons. Then did a hack job of putting them back in?[/QUOT

Calipers were rebuilt ones from Rock Auto. I think they were Centric rebuilds. They were pressure tested when rebuilt so I'm pretty sure seals were ok when installed.
 

Vetracr

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Mar 16, 2018
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I've been thinking through the problem. The piston floats on the seal. When braking the pad exerts a shear force on the piston, i.e, the piston is forced into the seal and twists so the back of the piston hits the bore wall. The two contact points counteract the shearing force in the piston caused by the pad friction. The effect is to locally crush the seal repeatedly in the same spot. I understand the mechanics but I don't understand the failure mode of the seal.
 
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Wow! That's scary. I have rebuilt countless OEM BMW and aftermarket calipers that have been used for racing and I have never seen this. Like @silverstreak18 said, what is the history of those calipers? Do you think they have been rebuilt before?

Two things come to mind. One is faulty seal manufacturing where an air bubble was lurking just under the surface and the surface gave way and exposed the void. Or... the calipers were rebuilt at one point and the pistons were pressed in with no lube on the piston or seal causing the seal to tear.

Anyways, yes, glad that didn't happen in the braking zone at 130 mph!
Barry
 

NoQuarter

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Nov 24, 2017
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Anyways, yes, glad that didn't happen in the braking zone at 130 mph!
Barry

Ha... I was coming off a 120 mph straight with my driving instructor, braking way too soon and with an on/off/on the brakes. He asked me what I was I doing.... " I don't trust my stock brakes! " 90deg turn at the end!
 

Vetracr

Private
Mar 16, 2018
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16
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BMW Z4
Wow! That's scary. I have rebuilt countless OEM BMW and aftermarket calipers that have been used for racing and I have never seen this. Like @silverstreak18 said, what is the history of those calipers? Do you think they have been rebuilt before?

Two things come to mind. One is faulty seal manufacturing where an air bubble was lurking just under the surface and the surface gave way and exposed the void. Or... the calipers were rebuilt at one point and the pistons were pressed in with no lube on the piston or seal causing the seal to tear.

Anyways, yes, glad that didn't happen in the braking zone at 130 mph!
Barry

Barry,

The calipers were Power Stop rebuilds from Rock Auto. I've used their stuff before and I've never had a problem. They pressure test calipers before they package them. Pads were worn but not enough to set wear indicators. I'm thinking Cool Carbon ST pads which really grab are causing pistons to pound seals under initial application of brakes when pistons are extended from bore. Seal sits maybe a mm or so above bore so it doesn't add up. If seal was compressed piston would hit bore. It just doesn't make sense. Time to start saving up for multi piston calipers.

Larry
 
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