A gearbox isolator (10) is provided to allow the gas within a gearbox cavity (50) to expand and contract in a manner that prevents the ingestion of airborne contaminants such as moisture and particulate matter. The gearbox isolator utilizes a breather bladder (22), a check valve (36) for venting gas to the atmosphere when the breather bladder is filled to capacity, and a desiccant filter (32) to provide make-up air to the gearbox when the breather bladder is empty. The desiccant filter is provided in a sealed canister (34) having a check valve (38) allowing atmospheric gas into the sealed canister when needed.