I've had good success pushing the pistons out (after removing the pads) and giving them a blast with some spray lube (I use weldtite TF2) pushing them back in and cycling them a couple of times, then clean up with isopropyl/brake cleaner and reinstall pads. Job done
I'd do that, then check the bleed.

(This is my "quick fix" technique I use at work on a daily basis, I have only found a handful of brakes it hasn't sorted: and they were found to be beyond repair)

Brake seals can easily be ruined from contact with common lubes and cleaners, what's OK for mineral oil systems will fry glycol seals and vice versa.

I reckon it's a port timing issue in the levers