What was found
AFTER TAKEOFF, PRESSURIZATION MALFUNCTION OCCURRED. ACCOMPLISHED ABNORMAL PROCEDURES. AC RETURNED TO FIELD. LANDING NORMAL. MX FOUND GROUND VENT VALVE STUCK CLOSED. MX CLEANED AND RESET GROUND VENTING VALVE IAW 21-31-17, PG 501. OPS CHECKED SATISFACTORY. AIRCRAFT RETURNED TO SERVICE. (K)