So I've just gotten a new bike which has disk brakes, and it's my first time using disk brakes. One of the brake pads is rubbing a little on the rotor, so I decided to turn the barrel on the lever anti-clockwise to "take some tension off the cable" (bear with me).

When I pulled the lever and fluid shot out I became aware of 2 things:

  1. there is no "cable" because my brakes are hydraulic.
  2. I'm an idiot.

The brake still works but now that there's fluid missing do I need to top it up?

(I also need to learn about hydraulic brakes).

  • 1
    If those are Avid-s, DO wash the handlebars with a lot of water. The fluid is corrosive and toxic. If those are Shimano-s, this is not a concern.
    – Vorac
    May 30, 2016 at 11:02
  • 1
    Should have mentioned, they're Shimano BR-M355
    – frenchie
    May 30, 2016 at 11:21
  • Those are good brakes for the money. You could 1) upload a picture of which exactly bolt did you unscrew 2) tighten it back before riding and 3) report if there are any issues with the brake now. If there are no issues, I wouldn't worry (hydraulic brakes are designed to tolerate small problems of this kind). Although, keep in mind that as the pads begin to wear down issues could arise. The solution is a procedure, called bleeding - any technician can do it, at my country it costs 5 EUR.
    – Vorac
    May 30, 2016 at 11:45
  • All you need is spending time on sheldon brown sites on disc brakes sheldonbrown.com/disc-brakes.html
    – mootmoot
    May 30, 2016 at 12:06
  • 1
    +1 for learning number 2! Many cyclists would benefit from that same piece of experience :) Also before you find this one out by experience, never squeeze the brake lever without the rotor between the pads. Guess how I found that one out!
    – Criggie
    May 31, 2016 at 7:38

1 Answer 1


Your brake is a closed system, and your brake cable is filled with a mixture of brake fluid and air. Air is compressible, while brake fluid isn't. So, the larger the proportion of air in the system, the more compressible it will be, the more effort is required to actually stop the bike, and this is frequently described as "spongier".

Now, in your specific case, you need to judge whether you have enough stopping power for the environment/conditions in which you ride. If the answer is "No", then topping up the reservoir (i.e. replacing air with brake fluid) might make sufficient difference.

  • If there's air in the system, it may no longer be in the reservoir, but in the brake lines. The system will appear full, but won't be and you'll get the sponginess. The only solution for that is bleeding, as Vorac mentioned.
    – FreeMan
    May 31, 2016 at 18:48
  • There should be almost no air in the system. If there is air then brake performance is compromised and may be variable. It is fairly simple to bleed the brakes yourself, but follow the correct procedures, since brake fluid can be caustic. (Or have a shop bleed them.)
    – Lee-Man
    Jun 1, 2016 at 16:30

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