isn't it possible to add more fluid without completely bleeding and replacing?
Yes, you can. Just bear in mind that if you do this with part-worn pads, you might not be able to retract the pistons far enough when you want to install new pads, without bleeding some fluid out of the caliper.
@Darkerside, you might need more than acetone. Discs do have a sort of 'pad memory' in the form of the microscopic glazing and pad material build up. A better option is to take a sanding block and 400 grit wet-and-dry, and give the disc a good going over in a small circlular motion, removing all of the shine, then clean with disc brake cleaner (I use Muc Off's stuff in the black aerosol). New pads will also need a couple of dozen really hard stops from about 20mph to properly bed in with the disc. Every few stops, pour cold water onto the discs (and marvel at the amount of heat you've generated).
But good performance that returns to poor performance does suggest a caliper problem; either air is getting in past the piston seals, leading to spongey behaviour, or fluid is getting out past the seals or from between the two halves of the caliper body and ruining the pads (my XTs are a split design (so far, so good); and my old Hope Minis were like that: the rear used to leak onto the pads and the braking felt like the pads were made of soft plastic). You could try sanding the pads and the dousing in the Muc Off brake cleaner; it can rejuvenate things if you're lucky.
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