You will need to bring the pressure back up on the guage by turning on the tap on the flexible filing loop until the guage reads one bar. Each time you bleed the radiators, you willl have repeat this process. You may also have a slow leak in the system? keep an eye on the pressure and see how long it takes to go down.
Sometimes the pressure relief valve on the boiler leaks,you will know if this is happening by looking at the small copper pipe outside. if there is some water coming from this pipe - the relief valve will also need to be replaced.
SPS
January 2009