What was found
WHILE CLIMBING THROUGH FL 220 THERE WAS A FAINT POP, FROM THE PRESSURIZATION SYS. THE ACFT STARTED TO LOSE CABIN PRESSURE, THE FLIGHT CREW INITIATED A DESCENT TO A LOWER ALTITUDE. THE CABIN ALTITUDE RECHED APPROX 6,000 FT THE MASKS DID NOT DROP. ONCE THE ACFT WAS STABILIZED AT THE LOWER ALTITUDE THE PRESSURIZATION SYS RESPONDED NORMALLY. THE ACFT RETURNED TO DEPARTURE WITH 3 PASSENGERS AND LANDED WITHOUT INCIDENT. THE CAUSE OF THE FAILURE HAS BEEN IDENTIFIED TO A WEAK NACELLE BLEED AIR MIXING VALVE ON THE LT SIDE.