Help with Giant MPH Brakes

[militantmandy]

Hi folks, wondering if someone could help me with a problem i've got. Over the last week or so, the rear brake on my Giant XTC 4.5 (Giant MPH hydraulics) has been gradually loosing pressure, to the point that it's now basically useless. Can anyone guide me as to what to look for/check to try and work out what's wrong with it?