An air turbine starter configured to be coupled to a gearbox, the air turbine starter comprising: a turbine housing (12,13) including an opening adapted to be in flow communication with an interior of the gearbox; an output shaft (70,90) extending from the turbine housing (12,13); a cover plate mounted to the turbine housing (12, 13) adjacent the turbine output shaft, the cover plate including an opening adapted to be in flow communication with the interior of the gearbox; a first pressure valve (100,118, 142,158) disposed in the opening of the turbine housing (12,13); and characterised by: a second pressure valve (100, 118, 142, 158) disposed in the opening of the cover plate, wherein: the first pressure valve and the second pressure valve are each configured to remain open when a pressure differential between a first pressure in the turbine housing (12, 13,) and a first pressure in the gearbox is below a predetermined level, and the first pressure valve and the second pressure valve are each configured to close when a pressure differential between a second pressure in the turbine housing (12, 13) and a second pressure in the gearbox is above a predetermined level to prevent oil from the gearbox from entering the turbine housing. a turbine housing (12,13) including an opening adapted to be in flow communication with an interior of the gearbox; an output shaft (70,90) extending from the turbine housing (12,13); a cover plate mounted to the turbine housing (12, 13) adjacent the turbine output shaft, the cover plate including an opening adapted to be in flow communication with the interior of the gearbox; a first pressure valve (100,118, 142,158) disposed in the opening of the turbine housing (12,13); and characterised by: a second pressure valve (100, 118, 142, 158) disposed in the opening of the cover plate, wherein: the first pressure valve and the second pressure valve are each configured to remain open when a pressure differential between a first pressure in the turbine housing (12, 13,) and a first pressure in the gearbox is below a predetermined level, and the first pressure valve and the second pressure valve are each configured to close when a pressure differential between a second pressure in the turbine housing (12, 13) and a second pressure in the gearbox is above a predetermined level to prevent oil from the gearbox from entering the turbine housing. a second pressure valve (100, 118, 142, 158) disposed in the opening of the cover plate, wherein: the first pressure valve and the second pressure valve are each configured to remain open when a pressure differential between a first pressure in the turbine housing (12, 13,) and a first pressure in the gearbox is below a predetermined level, and the first pressure valve and the second pressure valve are each configured to close when a pressure differential between a second pressure in the turbine housing (12, 13) and a second pressure in the gearbox is above a predetermined level to prevent oil from the gearbox from entering the turbine housing. the first pressure valve and the second pressure valve are each configured to remain open when a pressure differential between a first pressure in the turbine housing (12, 13,) and a first pressure in the gearbox is below a predetermined level, and the first pressure valve and the second pressure valve are each configured to close when a pressure differential between a second pressure in the turbine housing (12, 13) and a second pressure in the gearbox is above a predetermined level to prevent oil from the gearbox from entering the turbine housing.