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The other day I was watching a video of a guy changing brake pads, and the first thing he suggested was opening the brake fluid container to remove some fluid with a syringe. This is to avoid spillover when one pushes the pistons back in place, provided one topped-up the oil as pads were wearing down. This got me wondering.

I've always used the brake fluid level as a first-inspection indicator of the wear of my brake pads. As the level drops I take a closer look at the pads to see how much "meat" is left on them. For this reason I rarely top-up the brake fluid. I always assumed that given a new disk and new pads, filling up the container to the required level ensures that there is the enough fluid for correct operation of the brakes. As the fluid goes down, the brake lever gains a bit of play, which I usually counteract by using the appropriate lever regulation system.

Is this bad practice?

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    A better idea would be to loosen the bleed nipple and let the extra fluid come out as you push the piston back. It is not a good idea to push the old fluid from the cylinders back into the brake system. This fluid will have regularly been exposed to the heat of the brake caliper and may contain fine particles from wear and tear on the piston/cylinder.
    – HandyHowie
    Apr 25, 2016 at 11:08
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    For as small as the system is, I've always used a pad change on a bike as an opportunity to flush the old brake fluid out and replace with new.
    – JPhi1618
    Apr 25, 2016 at 14:00
  • @HandyHowie That's really good advice and an excellent point. Wish we could +1 comments for points.
    – DucatiKiller
    Apr 25, 2016 at 18:38
  • Add fluid only if the fluid gets too low (below the low mark) in the reservoir during the life of the pads.
    – Moab
    Apr 26, 2016 at 22:56

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Er, not so much. I'm assuming we're talking Motorcycle here, right?

  • What if there is a slight leak in the brake lines somewhere? You'd be losing fluid and that would NOT be a good indicator of brake pad wear.
  • Different vehicles have different size brake actuators, with accompanying different volumes of fluid to account for as the brake pads wear down over time.
  • It would certainly be possible to have to add brake fluid to a master cylinder to accommodate brake pad wear, while there is still a whole lot of brake pad life remaining.
  • You really don't want to let the fluid in the reservoir run low. That would introduce an air bubble into the system, and that's just bad.
  • "A bit of play" er... no. A bit of play indicates worn master cylinder or worn wheel cylinders that don't operate at full potential (or you've got an air bubble in the line). Your brakes should operate with the same throw when new as when the brake pads are worn 50% (although in reality, my bike is the same way... I'm just too lazy to rebuild all the cylinders in the system.)
  • If you replace brake pads with new pads, you've got slight wear in the system, you will continue to see "a bit of play". The way to get rid of the play is to rebuild the cylinders and bleed all the air out of the system. Note: I can see a degradation in my brake system, as observed as extra lever travel, after 10K miles or so.
  • How do I know that? I've got two identical bikes... mine has lots more miles than the wife's... Her brakes are awesome, mine, not so much.
  • I will say, I don't really understand why the master cylinder and wheel cylinders on the motorcycle wear out so quickly. Insert break here... I just ran out to the garage with a magnet. Both the master cylinders and the wheel cylinders on my bikes are made of cast aluminum, not steel. Well no wonder they wear faster. Duh. The same parts on my car are made from steel or cast iron. Ouch.
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  • Where do you get the assumption that he's talking about motorcycle? Not saying you're wrong (and your answer is excellent regardless), I just didn't pick up on that.
    – loneboat
    Apr 25, 2016 at 15:45
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    @loneboat The question is tagged with motorcycle.
    – DucatiKiller
    Apr 25, 2016 at 18:39
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    ::facepalm:: Okay, my bad. :-D
    – loneboat
    Apr 25, 2016 at 20:04
  • So is this a yay or a nay on the topping up? ;)
    – JoErNanO
    Apr 28, 2016 at 13:10
  • @JoErNanO dunno. Who even cares. If that's your question you totally missed the point. In this case that means you didn't read my answer, and if you did you didn't comprehend the though process. The answer is understand why the system works and take appropriate action. The key here is that brake fluid level will appear to go down as the brake pads wear, and that you never ever ever want the fluid to get low. Beyond that you can do what ever you want to. Minimum fill required, obviously. If you want to top it off, go for it. Does it matter to the functioning of the brakes, not one bit.
    – zipzit
    Apr 28, 2016 at 17:37

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