What was found
APPROX 20 MINUTES INTO FLIGHT WHILE CRUISING FL290 MASTER CAUTION ILLUMINATED W/L HYD PRESS LOW LIGHT ON. ANNUNCIATOR PANEL HYDRAULIC PRESSURE WAS ZERO AND QTY WAS DOWN TO 2 QTS. RAN APPROPRIATE CHECKLISTS (SHUTTING OFF THE L ENG HYD PUMP), PRESSURE STAYED AT ZERO AND QTY CAME BACK TO 7 QTS. CHECKED LT ENG PUMP CASE DRAIN FILTER DUE LOW HYD QTY. FILTER CHECKS NORMAL. REPLACED LT ENG PUMP CASE DRAIN FILTER. REPLACED LINES AT LT PRESS/CASE RETURN MOD. PRESSURE CHECKS GOOD LEAK CHECK GOOD.