FWIW:
on most mobile equipment, the steering has "priority" over other powered
circuits.
the fact that the deck lifts, says your hyd. system is functioning but there is a problem with the steering circuit when commanded to steer, which you know.
With the engine running
Slightly loosening the hoses one at a time at the cyl, then turning the steering wheel L/R that should tell if the fluid is trying to advance /retract the cyl.
Wondering if there was a problem with the cyl blowing by internally, and then the hose gave way?
Disclaimer:
I've never touched your style mower, just giving generic experience.
oh, have a drain pan to catch the fluid it can have maybe #800 pressure at full turn, and of course wear eye protection.