I have a Giant Fastroad SL-1 with brakes listed as Tektro HD-R210 hydraulic disc, 140mm
Recently I bled the brakes and afterward noticed that there's a very slow leak coming from the bleed port.
The brakes had become very soft before the bleed - with full pressure applied I could just barely make the lever contact the handlebar.
What are my options?
Is repair feasible & cost effective? If not, what should I be looking for in a replacement?
Bleed port bolt got loose, I think. Most people do not tighten them enough, for a good reason: steel bolts on aluminium holes is a disaster waiting to happen - it's easy to strip treads.
I suggest applying blue Loctite to bleed port bolt. That way, you don't need to tighten it too much, risking treads. It will also fill treads, and lower the risk of oil leak.
Also try changing the bolt, if you can. It can be a sub-standard one, slightly thinner than the standard, or elongated for some unknown reason. It really doesn't make any sense that bolt is getting loose.
I would begin by removing the bleed nipple in order to inspect it and the seat it tightens against. Some calipers use a rubber sealing element (an O-ring of some sort), while others use two precisely-machined metal surfaces tightening against one another. Thoroughly clean the nipple and its seat to remove any dirt which may be interfering with the seal.
On the topic of repair, there isn't much you can do beyond cleaning and refitting the bleed nipple. A steel bleed nipple screwed into an aluminum caliper body is most likely going to result in the caliper being damaged (if the system uses a metal-on-metal seal). Not much you can do to fix it then.
Overall, assuming the leakage rate is small, this shouldn't be a safety-critical problem. Pretty much all hydraulic brakes leak oil and ingest air to some extent (it's why you need to bleed them in the first place). This specimen just does so really fast. You'd of course need to bleed them more often than usual though. Note that it wouldn't be expensive to purchase a new caliper (<$50) if you want that peace of mind. The used market is of course an option too--it's pretty common to see standalone calipers.
I'd consider replacing the caliper. I'm all for bodge repairs that work, but brakes are just too important.
You might get away using this caliper on the rear for a while, and use an oil-resistant sealant to set the bleed bolt in place permanently for the last time.
Checking for O rings and replacing any found is a good test as well - if you can find exactly the right O ring in the right material. Also check the other caliper visually and see if it is threatening to leak. Maybe this model of caliper is a bit weak here.
A helicoil may work, but there's not a lot of metal or depth to work with. Also the replacement threads are not as fluid-proof as cut threads, with more cavities.
Safety says replace the caliper and use a torque wrench next time.
