BRAKE PROBLEMM!! HELP

[geezy]

ok so about a month ago i changed a rear caliper, bled the system, seemed fine, about two weeks ago the pedal kept sinkin to the floor, i bled alll four wheels, the fluid came out with pressure, still kept doin it,and now while im drivin n try braking "sometimes" it sinks by itself n i lose brakes n i gotta pump it in order to get the right brake amount, now it doesnt sink anymore while pressin the pedal at a light like it used too, the pedal is hard, it only sinks while drivin or brakin to slow down, does it mean it still has air, or is it the Master Cylinder??

Thanks I.a!

[94_teg_ls]

sounds like the mc to me