What was found
IMMEDIATELY AFTER TAKEOFF, EQUIPMENT COOLING VALVE LIGHT ILLUMINATED WITH FWD EQUIP VALVE EICAS MESSAGE. ALSO, HAD CABIN ALTITUDE OF 6500 FT AT 13000. ACFT ALTITUDE WITH CABIN RATE FLUX FROM -250 TO -750 FPM WHEN LEVEL. CABIN ALTITUDE DESCENDED TO 5000 FT WITH ACFT AT 10000 FT. ACFT DESCENT TO 5000 FT AND CABIN ALTITUDE AT 0 FT. RATE FLUCTUATIONS CONTINUED TO LEVEL 5000. RETURNED TO DEPARTURE. REPLACED RELAY K336 INBD SUPPLY VALVE RELAY PER WDM 21-58-12. OPS CHECKED NORMAL.