high pressure raising to cut off (combi boiler ) ?

hi there i had a problem with my hot water, on my ideal responce 120 combi boiler i diagnosed it . i replaced the flow switch , to do so i had tro remove the pump. anyway when i put it all back together no leaks, i presurised the system to 1.5 bar, however when i turned the boiler on it fired up ok and was running well , but the pressure gage went us and made a LOT noise, then hot steam came out of the top, through the air vent, then the boiler shut down . the temp gage went , back down i tried it several times the same thing keeps on happening . its somthing to do with pressure build up, it didnt happen before its been ok for three years, does anyone know how to fix it please thanks joe