I have the older G1 style caliper Hayes HFX Mag HD. My rear brake is messed up. When I grab the lever, there's pressure, and the pads grab the rotor, but if I hold the lever down, the pressure goes down, and the pads back off, this continues until the lever is all the way down to the bar, and at this point the pads just barely contact the rotor. If I let go of the lever and grab it again, the pressure is back momentarily before slowly backing off again. I'm not sure if it's just a pocket of air, but it feels as if fluid is leaking out somewhere, or forcing itself back into the master cylinder. Any ideas?