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I have a simple Shimano hydraulic disc brake system (don't know the model).

Over time, the feel of brakes is getting soft, I have tried to adjust by tightening the screw (on the joining of brake line). I have also recently changed the brakes pads myself.

Is softness due to a decrease in fluid level?

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    If the brakes are getting soft it may be time to bleed the hydraulic system. – ebrohman Oct 16 '15 at 1:41
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    Could you please get the exact model number? Some have self-bleed functions and some need a nut released. Also, how long has the brake fluid been in there? Is it DOT or mineral oil in the brakes ? – Criggie Oct 16 '15 at 2:44
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    @Criggie, Shimano uses mineral oil only. – Klaster_1 Oct 16 '15 at 5:11
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    @Klaster_1 Thanks I was unaware (never owned a bike with hydraulics - I'm referencing my car knowledge.) Hopefully OP has the correct brake fluid. – Criggie Oct 16 '15 at 7:16
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    @DCODE, bleeding is done in order to remove air from hydraulic system. Air is easier to compress than oil, so any excess will decrease performance. The operation is pretty simple, you basically circulate oil through system with a syringe, funnel and a bag until (almost) no air remains. – Klaster_1 Oct 16 '15 at 8:23
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You should bleed the system with mineral oil, not top up. If it's your first time you shoud see a tutorial video first on utube . if there is no leak you don't need to tight anything. if you changed the pads you must brake at least 50 times on a safe place at 25 km/h to get the full braking power.

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