take the pressure sensor off again and read the markings on the spanner flats, there are 0.3bar and 1.8bar versions make sure you have the right one. as ben says check where you fitted it too, the 1.8bar one goes on filter housign 0.3bar on side of head.
buzzer is controlled by 1.8bar switch, with a yellow wire. the most common vdo pressure sensor is the 0.3bar one, so if you had a 0.3bar one and fitted that in the filter housing with a yellow wire then that would explain your problem