Had to replace a servo on diesel S-Type once before as was corroded and leaking pressure.
Fitted another one (later type as used on XF) and worked fine.
However, there is a actuating pushrod between the brake pedal and the servo which I believe it adjustable so if the replacment one is not set correctly then this could be the issue.
Would necessitate removal of the brake pedal pushrod (from the underside of the dashboard) and making an adjustment to the length which would release the hydraulic pressure off the braking system.