You can probably check to see if the control piston is leaking with a dental mirror just after shutting the engine off.
If leaking, the part number of the control piston seal ring is A 011 997 71 48. You will also need a A 004 997 06 48 seal between the fuel distributor and housing.
The procedure is covered in the FSM under 7.3-1660.
It is critical to get the screw plug set to the same depth as before.