The deposits are usually corrosion from water that is absorbed into brake fluid.
The corrosion in the seal groove is the main cause of pistons not retracting as pressure is taken off when brake released.
I usually use an a shaped tool to scrape out and finish off with 3m type abrasive pad, cut to allow it to be pushed into grove as you turn.
Make sure you clean carefully blow out with compressed air then with brake cleaner and paper towel, then brake fluid on internal areas and use special brake rubber grease on seal as petroleom based grease will swell the seals.