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How much effect does low temperature and high altitude have on your brakes. I notice that my brakes get spongy at higher altitude and/or cold weather. I have only noticed this with brakes with mineral oil.I am guess that its a bit of air in the lines, that isn’t noticeable in normal conditions. Any thoughts, I don’t want to bleed them if this is normal and bleeding won’t cure it.
 

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How much effect does low temperature and high altitude have on your brakes. I notice that my brakes get spongy at higher altitude and/or cold weather. I have only noticed this with brakes with mineral oil.I am guess that its a bit of air in the lines, that isn't noticeable in normal conditions. Any thoughts, I don't want to bleed them if this is normal and bleeding won't cure it.
What do you consider high altitude?
 

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High altitude can have dissolved gas come out of solution which has the potential of making brakes spongy. Standard practice with Sram DOT brakes is to outgas the fluid by applying a vacuum on it with the syringes just before bleeding the brakes. When you do this, you see gas come out of solution. I've taken to doing this with mineral oil as well and can see significant gas come out of solution when I do it. IDK how this would affect your situation. I haven't experienced any problems up to ~ 9k'.

The issue I've had with mineral oil brakes in cold temps (below ~30F) is sluggishness caused by the increase in viscosity. Glycol based brake fluid (DOT) doesn't exhibit this problem.
 
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