Whilst replacing my pads I made the mistake of pulling the brake lever, which pushed the piston too far, and thus lost loads of fluid.

I've searched online for the type and quantity of fluid to replace it with, but I cannot find anything specific to my Specialized Hardrock.

  1. What kind of fluid do I need?

  2. How much will I need for one brake system?

  3. I've included picture of top and bottom of system for conformation of fluid in and out? Fluid in, I think; fluid out.

  4. could someone point me in direction of a video on replacing the fluid on this specific bike?

  • Welcome to the site! Like most of Stack Exchange, we don't give recommendations on products or suppliers, so I've edited that part out of your question. Aug 31, 2018 at 17:57
  • Also, note that the make of your bike shouldn't make any difference to the answer. Things like brakes and gears are almost always bought as components by bike manufacturers: they usually don't make their own. Aug 31, 2018 at 18:06
  • If its your first time bleeding, expect it will take longer, have to do it a couple of times, and you will spill more fluid than the internet videos suggest. Don't worry to much if you don't get right first time....
    – mattnz
    Sep 1, 2018 at 1:40

2 Answers 2


You have Tektro HDC 300 hydraulic brakes.

Follow the bleed procedure as described in the proper Tektro manual; https://www.tektro.com/upload/Product/F_20180828170966wPjDlH.PDF

The bleed process also adds additional fluid to the system so that will replace what you lost.

  • Thanks for the link it helped. I'm not asking to recommend product, but will this: epicbleedsolutions.com/products/bleed-kits/tektro-trp/… 1) have the correct type of oil to top up what I lost? 2) have the specialist tools needed to do the job?
    – reggie
    Sep 1, 2018 at 7:51
  • Also is that blue plastic tool a 2 piston bleed block?
    – reggie
    Sep 1, 2018 at 8:03
  • @reggie Based on what the web site says the kit is compatible with your Tektro brakes. It includes a high quality mineral oil which is what Tektro uses on their models. I don't know what you mean by 'two piston' bleed block. AFAIK all hydraulic brakes use two opposed pistons. If you want to be really sure the kit works for HDC 300 model, ask the manufacturer (they have a 'contact us' link on the web site). Sep 1, 2018 at 11:29

For your Tektros you would need "Mineral Oil" fluid. It's NOT THE SAME a DOT Fluids, don't mix them. Urban legend says that would make them go kaboom.

To replace the fluid:

  1. Hang your brake, in a way that the caliper is at the bottom and the lever is leveled at the top. Work while is on the bike stading up.
  2. Put a tube at the the caliper, remove the rubber cap that you can see in the picture you uploaded.
  3. Put the other end into a bottle with a little bit of mineral oil. This way it would suck up mineral oil instead of air.
  4. Put something beteween the pistons, like a rotor, or a pad separator. This is important because you will need to pump the lever a few times and you don't want your pistons moving around.
  5. Open the top and have you oil ready to pour.
  6. Hold the brake lever, softly, it need to bit you separator.
  7. Open a little the caliper end, where you attached the tube. You'll see and feel the oil flowing.
  8. Top up the oil in the reservoir and do it again a few times. You will see a change in color of the fluid going out, and you should not see any bubble.
  9. Repeat till you are happy with it.
  10. On your last pump, close the caliper end while the lever is pressed. This way it won't suck any air, or old oil, from the tube or bottle.
  11. Top up the fluid and close the reservoir.

This's was the way I did it on brakes like this, always worked. If you still have bubble in your system. With everything closed, hang it fully upside down and vibrate it a little. So the bubbles will go to the caliper. Then put a tube a the caliper end, press a little the lever, open up the caliper, don't depress the lever and close the caliper again. Air in the system should be trapped in the tube mixed with a little bit of oil. Now you should be almost without bubbles.


PS: Brake fluids are nasty, use gloves and cover everything where you are working.

  • 1
    I'm going to have to point out that this is POTENTIALLY DANGEROUS as it is not the bleed procedure described in the Tektro manual, and therefore may not fill the brake system nor bleed the system of air properly. Don't do this, follow the procedure described in the manual - link in my answer Aug 31, 2018 at 17:51
  • @ArgentiApparatus I'd not go as far to call it Dangerous, sketchy maybe. Used this procedure on Tektros and Shimano XT, even though manual had their own procedures. This is like automotive standard bleeding. Never had a hicup.
    – dmb
    Aug 31, 2018 at 17:56
  • 1
    @dmb Doing anything sketchy to your brakes is flat-out dangerous. Aug 31, 2018 at 17:59
  • 1
    @dmb "sketchy maybe" "There nothing sketchy about this". Make your mind up. Aug 31, 2018 at 18:04
  • 2
    Any responsible contributor will always refer to the manufacturer's manual (or direct to a professional bike mechanic) when dealing with safety critical item such as brakes. Aug 31, 2018 at 18:10

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