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What are the factor(s) that would make the brake lever of a hydraulic brake become stiffer over time?

By stiffer, I mean that the lever becomes more difficult to pull, it offers more resistance and I need to exert more force in order to apply the brake.

By time, I am thinking a month or so following a service, with only moderate riding in the meantime.

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    My mineral oil Shimano BR-M615 get stiffer depending on air temperature. When it reaches about -20C, there's almost no lever throw. So, here's one such a factor for you. – Klaster_1 Dec 4 '15 at 14:31
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    Thanks @Klaster_1. But do they go back to normal when it warms? That seems to suggest that viscousity is coming into play, which seems fair enough – PeteH Dec 4 '15 at 15:07
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    Are these mineral or DOT brakes? You'll know from the other question regarding Elixirs that water absorption into DOT fluid will increase volume and so have a stiffening effect on the lever. – Bikebum Dec 4 '15 at 18:00
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    @User3875425 I'm interested in either fluid, I'm trying to understand all the possible reasons why this could happen – PeteH Dec 4 '15 at 18:11
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    Water and temperature like Klaster_1 said are definitely two reasons. I'm also thinking that seals inside the caliper/lever of DOT brakes expand over time I think I read that somewhere..could be wrong though! – Bikebum Dec 4 '15 at 18:18
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+100
  1. Brake fluids are more viscous when cold
  2. Water absorption can increase the viscosity

But only until heat causes the water to become gas, which is compressible. When this happens, the lever feels a lot less stiff and pressure at the pad is drastically reduced.

  1. Your system may have originally been poorly bled.

Tilting and vibration of the bike during use can encourage bubbles to travel upwards. Assuming you have an "open" hydraulic brake system, pressing the lever opens and closes a valve between the pressurised volume in the hose and the free fluid in the lever reservoir. Bubbles at the top are displaced into the reservoir through the valve until it closes, fixing the volume and allowing lever pressure to be transmitted to the caliper. When the lever is released, the valve has another open phase during which (hopefully) fluid is drawn into the volume. This behaviour is easy to observe in designs like Shimano's XTR BL-M975.

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