my vectra keep building pressure in the header tank?

hi ive had my head skimmed and pressure tested new head gasket and bolts,checked the header tank return pipes are not blocked and took rad off and flushed it through was perfectly clear checked water pump seems fine but still getting preesure in the header tank run it for 15 - 20 mins undo the cap on header and water rises and comes rushing out any ideas what it could be