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?
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.
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.
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.