If it is leaking oily fluid, it is the Hydro unit. However, if it is leaking brake fluid, it will be the seals in the mastercylinder.
It is best to replace these with new parts and not trying to rebuild, or install a used unit.
You can tell the difference-- place a little on your finger and try to wash it off with water only- brake fluid will wash off- Hydro fluid will not, without soap.
Chevy 04.5 LLY, 2X, shell, XDR, MikeL Heavy duty up-grade, 6spd, TransGo, deep pan, de-badged, 65 Gallon replacement Aero tank, LBZ-mp, Kennedy pump, shimmed, Pre-fuel filter, tow mirrors, BU cam, gauges, GPS, Sirius, bags, custom grille, fender vents, HD trans cooler- Curt front hitch, footwell lights, load lights, Studs, head gaskets.--- Jerry