What was found
NR 1 BLEED TRIP LIGHT ILLUMINATED PASSING 17000 FT MSL. NO DUCT PRESSURE SPLIT NOTED PRIOR TO LIGHT. BLEED TRIP RESET WAS SUCCESSFUL FOR APPROXIMATELY 5 MINUTES. BLEED TRIP LIGHT ILLUMINATED AGAIN PASSING 21000 T MSL. IAW QRH CHECKLIST, TURNED NR 1 PACK OFF. RETURNED TO DEPARTURE. ACFT GROUNDED . AIR TURNBACK - PERFORMED BLEED AIR HEALTH CHECK CARD NG-36WC-00001 STEPS 1-5. FOUND THE PRECOOLER CONTROL VALVE BAD. REMOVED AND REPLACED PCV IAW AMM 36-12-02. OPS AND LEAK CHECKED GOOD. COMPLETED TASK CARD NG-36WC-00001 STEP 6. CROSSOVER OPS TESTED GOOD IAW AMM 36-11-00-710-801.