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Shimano hydraulic disc brakes (BL-M396) leaking (I think) in very cold temps

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Shimano hydraulic disc brakes (BL-M396) leaking (I think) in very cold temps

Old 03-08-19, 06:04 PM
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robo
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Shimano hydraulic disc brakes (BL-M396) leaking (I think) in very cold temps

My winter bike is a 2015 Scott Sub Speed 10, which has Shimano BL-M396 hydraulic discs.
I commute daily in Minneapolis, where we have cold winters, with commuting temps sometimes below -5F (-20C).
I've started to believe that that at very cold temps, somewhere below 0F, the Shimano brake system starts to leak. Maybe it's the seals shrinking or perhaps going hard, but in any case, after exceptionally cold days, I seem to end up with air in the brake lines, and squealing, juddering brakes (and oily looking calipers and rotors).

It took a couple seasons of commuting to figure out why my brakes were acting up like this, and I had replaced both the rotors and pads in earlier attempts to fix the problem, thinking they were at fault. But now I'm fairly certain it's due to fluid leaks.

Anyone else experienced this? Are other hydraulic systems better suited for cold temperatures, or should I be looking to replace this with a cable system?

Cheers,
robo
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Old 03-08-19, 07:52 PM
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Mineral oil apparently cannot freeze solid, but gets pretty thick below 0C, to the point where it "gels." This would certainly explain your brakes behavior below zero. Brakes using DOT fluid will not experience thickening or freezing, but have to be maintained more regularly due to the hygroscopic nature of DOT fluid.
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Old 03-09-19, 08:45 AM
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Originally Posted by DrIsotope View Post
Mineral oil apparently cannot freeze solid, but gets pretty thick below 0C, to the point where it "gels." This would certainly explain your brakes behavior below zero. Brakes using DOT fluid will not experience thickening or freezing, but have to be maintained more regularly due to the hygroscopic nature of DOT fluid.
but the problems Im seeing seem to be the opposite. The fluid doesnt gel, it leaks out. So I suspect the fluid is still liquid but the seals are being affected by the cold.
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Old 03-09-19, 02:47 PM
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Leaking out from Where?

Sure the fittings did not loosen up?, snug them down a bit?

AFAIK , because my TRP Hy Rd brake has no hoses, master and 2 slave cylinders behind the pads,
are in the same part.. the diecast caliper, So no bleeding is to be anticipated ...
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Old 03-09-19, 04:27 PM
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I also had the leaking problem with shimano brakes many years ago. Had them rebuilt with new seals and it kept on happening. I think it was the combination of cold and road salt that damaged the seals That's one of the reasons why I changed to BB7s. I just hated dealing with leaking fluid.
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