Use brake fluid, DOT4. Any decent brand will do. Just take a fresh can, not one that has been open and exposed to air for some time. It is less than a liter that is required to complete flush the system. You should do that. Drain as much as possible (suck it out) and clear out any old fluid in the pump by pumping in new, also flush the slave cylinder and the lines. The diagnose tools have a feature to energize the PSD valve which you should do to pass on the new fluid and test the system. Be careful not to let any air enter the system, or its back to square one.
The slave cylinder has a 7mm Schrader valve just above the right side half axle. Bitch to reach but it can be done with the axle in place.
Bleed until fresh fluid without bubbles exit the bleeding hose.

Always work in a clean way, don't let any dirt enter the system. The fender is not the cleanest area, so be careful.

One more thing: the filter in the reservoir sometimes turns to rubbish or corrodes. It can be replaced as a spare part with not too much trouble. Any dirt that enters the high pressure pump (180 bar pressure so be careful) may damage the pump.