As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) DURING WALK-AROUND, A LARGE AMOUNT OF HYDRAULIC FLUID WAS FOUND ON THE INSIDE OF THE RIGHT HAND MAIN GEARBOX COWL. AIRCRAFT WAS RUN-UP AND HYDRAULIC FLUID WAS FOUND LEAKING FROM THE UPPER VENT HOLE. SUBSEQUENT INFORMATION REVEALED THAT LEAKAGE FR0M THE VENT HOLE WAS CAUSED BY THE PISTON ROD RUBBING AGAINST THE RING WIPER THEREBY CAUSING WEAR AND SUBSEQUENT FLUID LEAKAGE.