worcester 24i - too high and too low pressure?

hi.

i have a problem with my boiler. when i open that filling loop, it fills the boiler but the pressure goes up to 3 bars ! and it starts humming and making noises so i presume it starts draining to excess water. but when i close the loop - pressure goes down to zero and i have no central heating then. any ideas on that ? thanks

Thomas
November 2009