What was found
AT FL250 CABIN PRESSURE DROPPED TO -4000 PSI; ALT HORN SOUNDED. RAN QRH PRESSURE WAS CONTROLLED IN MANUAL. INSPECTED BOTH CARGO DOOR PRESSURE SEALS IAW MM 52-09-11, NO DEFECTS NOTED. PRESSURIZED ACFT TO 4.5 PSID, NO ABNORMAL LEAKS NOTED IAW MM 21-31-00. ALSO PERFORMED PRESSURIZATION DECAY TEST TWICE WITH DECAY TIMES TO BE 119 SECS. PERFORMED BOTH PRESSURE CONTROLLER BITE TEST IAW MM 21-31-01, & FOUND SEVERAL LEGS WITH LOW INFLOW, HIGH LEAKAGE FOUNDS ON SEVERAL RANDOM FLIGHT LEGS. R & R NR 1 ENG HIGH STAGE REGULATOR IAW AMM 36-11-07, LEAK & OPS CHECK DUE. R & R NR 2 ENG HIGH STAGE REGULATOR IAW AMM 36-11-07, LEAK & OPS CHECK DUE. R & R NR 2 ENG HIGH STAGE VALVE IAW AMM 36-11-06, LEAK & OPS CHECK DUE. R & R NR 1 ENG HIGH STAGE VALVE IAW AMM 36-11-06, LEAK AND OPS CHECK DUE. COMPLETED OPS CHECKS IAW AMM 36-11-06, & AMM 36-11-07.