What was found
PDX FLIGHT 2242: AFTER PUSHBACK AND AFTER START CHECKLIST (ENGINES 1 AND 2 BOTH UNFEATHERED) STANDBY PRESS INDICATED ZERO PSI STBY HYD PRESSURE SWITCH LIGHT WAS ON AND NO CAUTION/WARNING LIGHTS ILLUMINATED. NO C/B'S POPPED. CONDUCTED MEL PROCEDURE 29-30-2. FAILED ITEM NR 3 TEST. RETURNED TO GATE. MAINTENANCE REMOVED STANDBY POWER UNIT IAW Q400 AMM TASK 29-11-36-000-801. INSTALLED NEW SERVICEABLE UNIT IAW Q400 AMM TASK 29-11-36-400-801. BLED NR 1 A/C HYD SYSTEM. OPS CHECK STBY POWER UNIT SATISFACTORY. FLUSH NO REQUIRED DUE TO PARTICLE CONTAMINATION INDICATION (Q400 AMM TR 29-074).