A method for testing an annular seal within a gas turbine engine, the method comprising: mounting a tool to a structure of the gas turbine engine, wherein a first volume is between and is formed by the tool and the structure; applying a vacuum to the first volume through a conduit, wherein the annular seal is between the first volume and a second volume, and the conduit is connected to the first volume through the tool; measuring a vacuum pressure within the conduit while the vacuum is applied; and comparing the measured vacuum pressure to a threshold vacuum pressure, wherein a difference between the measured vacuum pressure and the threshold vacuum pressure is indicative of leakage across the annular seal from the second volume to the first volume, characterized in that: the tool extends circumferentially about an axis, and the tool includes a base, an inner flange and an outer flange; the base extends radially between the inner flange and the outer flange; the inner flange is connected to an inner end of the base, the inner flange projects axially out from the base, and the inner flange is configured to axially overlap and seal against the structure; and the outer flange is connected to an outer end of the base, the outer flange projects axially out from the base, and the outer flange is configured to axially overlap and seal against the structure. mounting a tool to a structure of the gas turbine engine, wherein a first volume is between and is formed by the tool and the structure; applying a vacuum to the first volume through a conduit, wherein the annular seal is between the first volume and a second volume, and the conduit is connected to the first volume through the tool; measuring a vacuum pressure within the conduit while the vacuum is applied; and comparing the measured vacuum pressure to a threshold vacuum pressure, wherein a difference between the measured vacuum pressure and the threshold vacuum pressure is indicative of leakage across the annular seal from the second volume to the first volume, characterized in that: the tool extends circumferentially about an axis, and the tool includes a base, an inner flange and an outer flange; the base extends radially between the inner flange and the outer flange; the inner flange is connected to an inner end of the base, the inner flange projects axially out from the base, and the inner flange is configured to axially overlap and seal against the structure; and the outer flange is connected to an outer end of the base, the outer flange projects axially out from the base, and the outer flange is configured to axially overlap and seal against the structure. the tool extends circumferentially about an axis, and the tool includes a base, an inner flange and an outer flange; the base extends radially between the inner flange and the outer flange; the inner flange is connected to an inner end of the base, the inner flange projects axially out from the base, and the inner flange is configured to axially overlap and seal against the structure; and the outer flange is connected to an outer end of the base, the outer flange projects axially out from the base, and the outer flange is configured to axially overlap and seal against the structure.