What was found
WITH NR 1 ENGINE RUNNING, TRYING TO START NR 2 ENG THE APU AIR DOES NOT RISE ABOVE 20 PSI - WITH ISOLATION VALVE SWITCH IN AUTO OR OPEN, DOES NOT AFFECT THE PRESSURE, STILL 20 PSI, MAKING NR 2 ENG SLOW TO SPOOL. ACFT GROUNDED. FOUND PRSOV ON NR 1 ENG DEFECTIVE. R & R NR 1 PRSOV REF AMM 36-11-04. RAN NR 1 ENG, OPS CK NORMAL. ALL ENGINES OPERATING NORMALLY, GOOD FOR SERVICE.