I have a shimano XT hydraulic disc brake system. While servicing it I took out the pads an checked the pistons in the caliper and noticed that only one is extracting. After a few pulls on the lever when the one piston reached the end of its travel, the brake fluid spurted from it. I've since resettled and reset the pistons but I'm thinking I might still have fluid leaking out from there in the future. Or maybe that was normal behavior?
That is normal behaviour. You pushed the piston way out, so oil from behind it found a way out.
General actions suitable for your case as well:
- remove wheel
- remove pads
- carefully push the lever a couple of times in order for the pistons to extend a couple of mms. if one is stuck, make sure you push the other one with a plastic tyre lever while pulling the brake lever in order to make the sticky piston extend.
- clean sides of piston with earbud with alcohol. clean again with paper. this ensures free movement of pistons later on.
- carefully fully push pistons back into their place with a plastic tyre lever. this is called "reset".
- insert shimano yellow block
- do a full bleed
- remove block
- install pads
- install wheel
- go ride
Also, make sure that no oil was spiled on pads or disk. If so, replace pads and clean disk very well.