What was found
HYD 3 LOW PRESS CAUTION MSG FOLLOWED QRH DIVERTED TO JFK 0 PSI IN HYD 3 80% QUANTITY IN HYD 3 HAPPENED 20 MINS AFTER TAKE OFF. AN EMERGENCY WAS DECLARED. PERFORMED TROUBLESHOOTING IAW FIM 29-12-00. FOUND ACPC HYD 3A BREAKER POPPED + NOT RESETTING. AFTER DISCONNECTING ACMP B15, BREAKER REMAINS IN. CONFIRMED WIRING GOOD TO ACMP 3A. REMOVED + REPLACED ACMP 3A IAW CRJ900 AMM 29-12-03. OPS CHK & LEAK CHK OF #3 ACMP 3A + 3B GOOD IAW CRJ900 AMM 29-10-00. VERIFIED 3B PUMP FUNCTIONS IN AUTO MODE WITH 3A OFF + ONE AC-GEN ON-LINE WITH FLAPS OUT OF 0 DEGREES. A/C OK FOR SVC.