10-06-15, 08:19 PM
The master cylinder seals could probably do with replacement if they're the originals still, but most likely it's just air in the system. The rear is a bastard to bleed because the path the fluid follows goes up and down so air can get trapped easily.
Taking the caliper off can help, as can cracking open the top banjo on the master cylinder to push out the air that gets caught there.