What was found
DURING ANNUAL INSPECTION, INSTRUMENT AIR PRESSURE GAUGE READ NORMAL WHEN EITHER ENGINE WAS RUN INDIVIDUALLY, BUT INDICATED HIGH WHEN BOTH ENGINES OPERATING. ALSO, NOTICED ACTIVATION OF DEICE SYS CAUSED INSTRUMENT TO 'PEG' FOR DURATION OF CYCLE. T/S SYS DISCLOSED ACTUAL PRESS DELIVERED TO GYROS WAS WELL BELOW NORMAL (APPROX 3.0 INCH HG) AND INSTRUMENT AIR REGULATOR SET TO MINIMUM, I.E., SCREWED ALL THE WAY IN. WHEN THE INSTRUMENT REGULATOR ADJUSTED TO PROVIDE CORRECT PRESS TO GYROS, ACFT PRESS GAUGE READING WENT 'OFF THE SCALE'. INVEST REVEALED SOURCE INDICATORS OF INSTR AIR PRESS GAUGE LEAKING & PRESSURIZING INSTR CASE INTERNALLY CAUSING ABNORMALLY HIGH INDICATIONS. DEFECTIVE INSTR REPLACED WITH O/H UNIT.