• Machine / system type — press, power unit, mobile circuit, etc.
• Symptom — what exactly happens, when, and since when.
• Fluid — ISO grade, condition, recent changes.
• Pressures & temperatures — gauge readings at rest and under load.
• What you already checked or replaced.
• Photos of the circuit, gauges or the failed component (no confidential drawings).

Good questions get expert answers fast. Vague questions get guesses.