Shimano MT500 leaking from bleed port My rear Shimano MT500 hydraulic disk brake has acting up for a while. After doing a full bleed and new set of pads it would be fine for about 2 weeks then start rapidly losing performance and screeching. I did this a couple of times to see if the problem would appear in the same way which it did, noticing oil on the pads each time.

I put a bit more time into bleeding the rear brake again - now when the brake lever is pushed fluid seeps out of the bleed port. This is happening even with the cup still on the lever and the system still open! Surely the pressure should be releasing from the top port which is open? 

During the bleed I noticed when I pulled back on the bleed port syringe there as a lot of small bubbles and it felt like no air would release and the pressure would snap the syringe handle back in, though it seemed fine when pushing fluid out towards the brake lever. Could something be blocked? 

I'm supposed to traveling with this bike on Saturday so I'm not sure if its worth just purchasing a new back brake now in case it's unlikely to be repaired on time? It's cheap enough to be worth it if required though I'd likely have to fit it myself in this timeframe, which I've not attempted before though does appear somewhat straightforward (though so did brake bleeding!).
It could be an easy fix, or a new brake. Are you prepared to trash your travel plans over the cost of new brakes?  Given the time frame, I would either hand it over to my LBS, or put on a new brake and sort it out when I got home. Time frame for sourcing a new brake means any hiccups and you won't have time to sort it unless you buy from your LBS. Your best option is probably get the LBS to do the work as well.    

If you replace the caliper on its own you will still need to bleed the brakes. Replacing the entire brake assembly, if you get one with the correct length hose, should require no bleeding. Shimano (Don't know abut others) ship with the hose detached from the lever so you can install into the frame if needed. Installing a new brake is much simpler than bleeding, especially if you run the hose on the outside of the frame. 

Only thing you really need to decide is what quality brake to buy. Your could go cheap - say MT365, with the plan to fix up the MT500's later, or use this as an opportunity to upgrade to M596 or better. (Going to M596's you should also upgrade the disc rotor to the narrow shimano version - MT500's use the wider Shimano rotor)

I recently needed a new back brake in a hurry and got discounted MT365's with a view that the $US20 cost could be saved by waiting for a discont on XT's. I still have not upgraded, the 365's are not great brakes, but for the rear give me enough power an modulation for my style riding.