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I recently bled my Tektro Draco 2 because it had become too spongy. Unfortunately, out of stupidity, I first opened one of the screws on the reservoir cap instead of the vent screw. It has been leaking since then and although I have cleaned everything thoroughly and reassembled it (the seal looks undamaged), I am unable to seal the cap again. I just saw that there is a small hole in the screw thread. I think this is where the oil is leaking from. Does this mean that the seal may no longer fit properly or is damaged?

reservoir seal hole in thread reservoir cap

zoom on hole in screw thread

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  • Mechanical screw threads don't normally seal - that's why the actual bleed screw has an O-ring (bottom left of pic. 3). But the screw hole is often where a leak manifests itself. I'd be looking for a damaged or misplaced gasket, perhaps on the back of the cap labelled "mineral oil"
    – Chris H
    Commented Jun 23, 2022 at 12:20
  • There are no other seals except the diaphragm itself (in the first picture laying on top of the tank) and the gasket for the bleed screw. I thoroughly cleaned the housing, seal and cap again and tightened it more. But I'm afraid this won't be the final fix.
    – conste
    Commented Jun 23, 2022 at 12:36
  • Is the brake still spongy? How much fluid is leaking?
    – Noise
    Commented Jun 23, 2022 at 12:53
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    The top gasket isn't really obvious in the photo, but spares are available, and this spare parts diagram shows one that looks like it surrounds the hole through which we can see fluid. Can you confirm that yours does? Does the gasket come off easily to inspect for damage?
    – Chris H
    Commented Jun 23, 2022 at 13:04
  • "Gasket" in the spares matrix I found, your "diaphragm"
    – Chris H
    Commented Jun 23, 2022 at 13:05

2 Answers 2

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In this area it's likely that any leak will be a fault in a rubber part or the reservor cover. These include the diaphragm and whatever o-rings are around the bolts, which appear to include the bleed screw only on this brake (not the reservoir cover screws).

Diaphragms are a part that do a lot of work and can sometimes fail even due to an apparently minor flaw.

Your brake has had this part available as a spare so I think your first and best plan is to try to obtain one: enter image description here

Barring that, you could either replace the brake or try to bring the diaphragm you have back to life somehow.

Flaws in the screw threads are not likely to be the issue, but you could test that easily enough with a different screw. In a system like this, the fluid pressure is being contained by the diaphragm acting as a gasket and providing sealing all around the parting plane between the cap and the reservoir, and all the screw does is provide the pressure needed to do that.

If you're getting a little bit of weeping oil from the pressure vent but the brake still functions, you could ride it a bit and see how it goes, on the premise that it could just be residual oil above the diaphragm.

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  • Thanks for your reply. Do you have any idea what the little hole inside the screw thread is good for? You can see it when closely inspecting the second picture. I guess its purpose is to let air pass for pressure compensation when the diaphragm expands when pulling the lever? I was really careful with the gasket all the time so I'm really wondering how this thing could get damaged if it did. Also I cannot see any cracked or brittle parts on it.
    – conste
    Commented Jun 23, 2022 at 17:33
  • @conste Could you add a zoomed photo with the hole circled? Perhaps I’m in need of glasses, but I can’t see the hole.
    – MaplePanda
    Commented Jun 23, 2022 at 17:39
  • @MaplePanda Sure, I added a zoomed shot. It's pretty tiny but definitely looks intended and manufactured like this. I suppose it's too let air pass and in my case only lets oil pass because the seal underneath it isn't working properly.
    – conste
    Commented Jun 23, 2022 at 17:44
  • @conste It's something to do with keeping pressure from building up where it's not supposed to be. Most or all open system hydros have a hole that does that. Shimano's look different but are there too. You can see the hole in the picture of the new one so I wouldn't worry about that.
    – Nathan Knutson
    Commented Jun 23, 2022 at 18:04
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    Yes, the hole is a vent so you don't get pressure built up on top of the diaphragm (especially if there's a bit of oil making a partial seal)
    – Chris H
    Commented Jun 23, 2022 at 21:06
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Received mine on 4 new bikes. Each bike had this problem with at least 1 lever. And one spongy lever with biggest leak.

This as new should NEVER have the fluid above the diaphragm/gasket. I found that gasket integrity had been compromised by over-tightening screw at vent. Cannot just get diaphragm/gasket for Tektro model HD-e350. So you get new screws, cover and diaphragm/gasket. (Only mfg part kit available for this model, makes you wonder if this is a consistent issue.)

Remember, depending on your riding type, with oil, dirt will collect, possibly get into vent/bladder system, plug vent (hindering proper activation). Just as with automotive brakes systems, separation of air/fluid must be maintained for safety. I would like to thank bike mfg for properly fixing.

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