Follow the instructions, and systematically separate the pump from the rest of the system. and identify which module leaks. The manual notes that you may not see liquid from a leak.

If it's the pump, they suggest possible causes as:-
Loose or leaky valve fittings.
Loose purge valve.
Contaminated purge valve.
Loose pump head screws.
Leaking seals or scratched plungers ( this would be my first guess ).
Leaking outlet valve.
Leaky damper.

Good luck,

Bruce Hamilton