What was found
LEVEL AT 13,000 MSL, RAPID PRESSURIZING OCCURRED. CABIN CLIMBED AT 6000 FPM. RATE OF CLIMB OR SOURCE SELECT KNOB HAS NO EFFECT. CABIN ATITUDE SELECTOR SET AT 400 MSL (BASICLLY SEA LEVEL). CABIN ALTITUDE STABILIZED AT AIRCRAFT ALTITUDE AND 13,000 MSL. OXYGEN MASKS IN REAR DID NOT DEPLOY. TROUBLESHOT TO L/H SQUAT SWITCH BEING OUT OF ADJUSTMENT. ADJUSTED SWITCH I/A/W 560 MM 32-61-00. RAN AIRCRAFT AND COMPLIED PRESSURIZATION SYS. OPS CHECK. OPS CHECKS GOOD. IAW MM 21-31-00. RETURNED TO DEPARTURE. EMERGENCY DECLARED.