if it is a rafter/over head tank shut off the make up water supply at the PRV - turn off the pump - if there is a valve leaving the bolier and entering the boiler close those - there should be a boiler drain midway on the expansion tank - open it - drain it until it stopps comming out of midway - close valve - open the two at teh boiler (make up water valve at the PRV) and start the pump. if expansion tank keeps flooding it sounds like your PRV is set too high - system should have 12PSI when the pump is off and the boiler is cool.

bladder type tank there is a shradder (bike tire) valve that pressure should be the same as gauge on booiler, shut pump off at amke up water - put bike gauge on bladder tank - there should be no water ever coming out of valve, if there is bladder is ruptured, if the pressure in the boiler is 12PSI check that bladder is at 12 and put air pressure in the bladder to 14PSI to push back bladder - and start system - they should equalize after some run time and bladder should center. - Jake