material handler

Hi we use a liebherr 934 4 cylinder 2006 material handler ,there is a problem with oil entering the cooling system ,we have had the oil cooler and casing witch it bolts to pressure tested it is OK .new gaskets were used when refitted .we also changed the front caseing where the water pump bolts to, due to some corrosion .could the head gaskets cause this problem any advice would be welcome thank you

material handler