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How does air get in closed hydraulic brakes?

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How does air get in closed hydraulic brakes?

Old 01-28-18, 01:48 AM
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DimplePimple
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How does air get in closed hydraulic brakes?

I have to bleed my Shimano hydraulic back brake. The bike only has about 2000km on it. It has been banged around a bit but I don't see any signs of damage.

This has got me wondering how exactly air usually gets in? I ride in the winter so could cold temp & contracting seals be a cause?
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Old 01-28-18, 08:27 AM
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brake fluid is hygroscopic. When brake fluid goes thru thermal cycles as it is in its nature to do so, moisture can occur. When that happens, the moisture degrades the hydraulic fluid, & if the fluid is not brought up to the ideal temps to bake out the water, the fluid retains the moisture & will keep accumulating more if the ability to cook out the water is not achieved. More water ppm to the brake fluid will increase the unwanted degradation. With enough water in the system, it will become noticeable during performance. Water can & will retain air during the aforementioned lifecycle of brake fluid. Meaning it will trap in air internally to the system.
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Old 01-28-18, 09:46 AM
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No system is truly sealed. Seals wear. Life happens and we must maintain or we break down. Andy
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Old 01-28-18, 09:52 AM
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Originally Posted by Troul View Post
brake fluid is hygroscopic.
Shimano system uses mineral oil.

I'd agree with Andrew - tiny bit of air getting past the seals over time. Not sure if cavitation could have something to do with it too - sometimes an issue with hydraulic systems.
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Old 01-28-18, 10:34 AM
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Thanks all. These hydraulics have turned out to be a pain. Bleed kit with oil has cost me $40, pads have cost me $20 & a new rotor has cost me $20 all under 2000kms. I've definitely learned a bit from dealing with these though.
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Old 01-28-18, 10:50 AM
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Only truly closed system Hydro brakes are Magura rim brakes .. they too use mineral oil.

no expansion tank.. because there is not the fluid heating of discs..


My HS 33 have only gotten new pads in 10 years.. of riding, never needed oil/ bleeding since I changed the hose lengths..






...

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Old 01-28-18, 01:17 PM
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Outside of small leaks, Im betting some is already trapped in the fluid before the system is filled, at least if the brake fluid isn't degassed properly.
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Old 01-28-18, 11:03 PM
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Why are you replacing your rotor? It should last through several sets of pads. Shimano recommends replacement when they measure 1.5mm thick. Even if its contaminated they can almost always be cleaned fine.

Most riders on Shimano brakes need bleeds annually or so. You can improve lever feel with a lot less work by top bleeding the system in between real bleeds.
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Old 01-29-18, 10:59 AM
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Originally Posted by cpach View Post
Why are you replacing your rotor? It should last through several sets of pads. Shimano recommends replacement when they measure 1.5mm thick. Even if its contaminated they can almost always be cleaned fine.

Most riders on Shimano brakes need bleeds annually or so. You can improve lever feel with a lot less work by top bleeding the system in between real bleeds.

That was completely unrelated to bleeding. Some idiot tried to steal it out of my hands & when he couldn't get it he kicked the rear triangle. Right on the rotor.
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Old 01-29-18, 06:51 PM
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Originally Posted by DimplePimple View Post
That was completely unrelated to bleeding. Some idiot tried to steal it out of my hands & when he couldn't get it he kicked the rear triangle. Right on the rotor.
What?! That's freakin' crazy!!! That aside, I love my Shimano brakes. I hated my Avids (which use brake fluid and have a $70 kit). I find the Shimanos easy to service and bleed. The kit is basically their little funnel, a hose, and a syringe--you paid $40? I can bleed each caliper in 25- 30 minutes. I do it only once a year. Pad-wise, it's two rears to one front for me--maybe once a year?
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