Hi,
Make sure you have a little play at the pedal to ensure the master cylinder is returning fully.
If this is okay you need to check if the wheel cylinder is siezing or rusting. There are 2 types, the earlier type was part of the casting under the linkage and is prone to water getting in and rusting it badly. The later type is a separate little cylinder under a plastic cover but still prone to rusting. The pins on the linkage sieze as well. Be careful not to twist the pipe off when you undo it. It will be much easier to access the assembly if you remove the wheel. They are not the nicest things to get to !! Don't adjust it too tightly when you finish as they will come on when you're going down the road.
Good luck
Mike