You have a hydraulic fan motor? Yes, they can leak either internally (like mine was) or externally (like it sounds yours is). What make of motor do you have? Maybe a seal kit is available, providing that the bores inside are not worn and that the gears still mesh tightly?
I have a Webster YC fan motor - seal kits for YCs are on eBay, but only for the pumps, not the motors that use higher-pressure seals. QCC is the current maker of YC pumps and motors, so I had them make me another exact replacement to go with my new Multi-Wing fan. My motor was simply not turning fast enough due to internal leakage, contributing to my bus's chronic overheating issues.