Detecting a leaking piston

I serviced my hope v4 caliper a few weeks ago (done the job several times before) because the pistons were sticking and not retracting.

Since the service something is leaking oil and contaminating the rotor.
It definitely wasn’t leaking/contaminated prior to the service
I didn’t disconnect the hose so I don’t think that connection can be the issue.
I did the paper towel trick in the bled port and that is fastened up tight so I don’t think that can be the issue either
I did have difficulty getting one position back in after I replaced the seals so I wonder if I’ve damaged the new seal in the process?
Anyone got any tips on identifying the source of the leak/problem piston?
I’ve got another seal kit so I could just replace all 4 but I’d prefer just to do one if I can get away with it

Cheers

James

Check the back of the pads for a ring of fluid?

Sounds like you have damaged a seal

Id remove the wheel and the brake pads then place/stick some tissue/kitchen roll over the piston you had difficulty  with and then wait and see if you get any traces of fluid on the paper

If it’s not obvious enough to see on the back of the pads then wrap a paper towel around a bleed block. Pull the lever until it grips the block, put an elastic band around the lever, leave for a few hours, see if you get a tell tale circle of fluid.

pdw+1