What was found
FLT NR 31 - EWR-MDW - AFTER TAKEOFF, COULD NOT MAINTAIN CABIN PRESSURE, RT PACK INOP. AIRCRAFT RETURNED TO EWR. FOUND NR 3 ENGINE 13TH STAGE MOD VALVE NOT OPENING WHEN RT PACK TURNED ON. REMOVED AND REPLACED 13TH STAGE MOD VALVE PER MM 36-12-42. NO HELP. FURTHER TROUBLE SHOOTING FOUND BROKEN WIRE ON RT PACK SWITCH. REPAIRED WIRE AND OPS CHECK OK AS PER MM 21-31-0. (M)