What was found
DURING CLIMB-OUT GOT ECAM YELLOW HYD SYS LOW PRESSURE. OVERHEAD QTY GAUGE INDICATED NO QUANTITY. AFTER LANDING QTY APPEARED NORMAL. COMPLIED WITH ECAM AND REFERRED TO QRH PROCEDURE HYD LOW PRESSURE. OPERATED YELLOW SYS WITH PTU, PRESSURE WAS 3000 AND QUANTITY WAS IN THE TOP HAT. OPERATED YELLOW ENGINE PUMP WHILE MOVING FLAPS AND ALL OTHER FLT CONTROLS, YELLOW PRESSURE WHILE MOVING CONTROLS DROPPED TO 2300 AT THE LOWEST. CONTINUED TROUBLESHOOTING IN PROGRESS. REMOVED AND REPLACED YELLOW SYSTEM HYDRAULIC QUANTITY TRANSMITTER. RAN ALL THREE HYD SYSTEMS AND LEAK CHECKED REMAINING FILTER, NO LEAKS NOTED.