1987 Other Mazda Models loosing water

hi i have a mazda e2200 made in to camper had new engine fitted but coolent light keeps lighting up ive watched it it is reaching temp pushing water out of radiater in to syphon tank over flowing ! i thought it was to draw water from syphon tank when hot and suck it out when needed !to me it is working back to front what do you sugest ps ihave put new rad senser in and new thermostat im lost help please