It probably needs new seals.

While its all in pieces take the pistons out and see what the seals look like, if they are swollen or frayed they need replacing.

Use this time to clean all the crap out from the seal grooves.

Once you have them re-assembled with new seals clean the crap off them regularly and they should be good for another 25 years

+1 for seals. I did mine recently. I thought the drag on my rear brake went away after a few seconds, but it turned out that even when I thought it had gone, there was still quite a bit, as with new seals the difference was ridiculous. Check the slide pins under the rubber boots too. Mine weren't too bad, but one of them is sufficiently tired looking that it will get replaced the next time I need to order some bits. A full set of genuine seals including the rubber boots is a bit under £20 delivered from DSS (other sources will be available). I did the pistons too. It turned out that they were probably OK to reuse, however they certainly weren't perfect and did have some light scoring on, presumably from crap trapped between them and the dust seals, which were diabolically swollen.