B737-700 CREW AND UNIDENTED ACFT HAD LESS THAN LEGAL SEPARATION IN LAX CLASS B AIRSPACE.

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FLT WAS DSNDING ON THE BASET 2 ARR JUST OUTSIDE OF REEDR INTXN. WE WERE DSNDING TO CROSS REEDR AT OR ABOVE 8000 FT. WHILE DSNDING OUT OF APPROX 9000 FT; WE RECEIVED A TA FROM AN ACFT CLBING. THE TA BECAME AN RA WITHIN SECONDS. THE TCASII SYS TOLD US TO MONITOR VERT SPD INDICATOR; BUT DID NOT TELL US TO CLB OR DSND. THE ACFT APCHED WITHIN -300 FT BELOW US ACCORDING TO THE TCASII DISPLAY. THE ACFT WAS SLIGHTLY OFF TO THE R OF US SO WE MADE A TURN TO THE L WHILE LEVELING OFF. THERE ARE SOME POINTS I WOULD LIKE TO BRING TO YOUR ATTN. ATC WAS IN THE PROCESS OF CHANGING OPS FROM THE E TO THE W BECAUSE RWY 7R&L HAD GONE BELOW MINIMUMS. WE WERE IN THE PROCESS OF BEGINNING OUR THIRD APCH PLATE BRIEFING WHEN WE RECEIVED THE TA AND SECONDS LATER THE RA. WE WERE IN IMC WITH MODERATE RAIN. IT IS MY OPINION THAT ATC WAS OVERLOADED WHEN THE RA OCCURRED. THE APCH FREQ WAS SO CONGESTED THAT WE WERE UNABLE TO INFORM THEM OF THE RA UNTIL WE GOT ON THE GND BY PHONE. WHAT I HAVE LEARNED FROM THIS EXPERIENCE IS WHEN YOU GET A TA; TRY TO FIND THE TA ON THE TCASII DISPLAY ASAP; BECAUSE AT HIGH RATES OF CLOSURE; THE TA TURNS TO AN RA WITHIN SECONDS. THIS IS THE FIRST TIME I HAVE HAD AN RA IN IMC AND I HAVE TO SAY; IT HAS GOTTEN MY ATTN. I WOULD LOVE TO SEE COMPANY TCASII TRAINING TAKE PLACE IN THE SIMULATOR. SUPPLEMENTAL INFO FROM ACN 565007: AT APPROX 2-3 MI FROM REEDR (APPROX 8500 FT); WE RECEIVED A TA (TARGET AT 1 O'CLOCK POS LOW; AND CLBING APPROX 800 FT BELOW). WITHIN SECONDS; THE TA CHANGED INTO AN RA. WE GOT THE 'MONITOR VERT DISPLAY' WARNING AND TARGET WAS APPROX 500 FT BELOW AT 1 O'CLOCK POS CLBING. VVI WAS RED -- NO GREEN BARS SEEN. LAX WAS LNDG E AND THERE WAS HEAVIER THAN NORMAL RADIO TFC DUE TO THE WX OBSERVATIONS GIVEN BY APCH CTL AND THE RESULTING INQUIRIES ABOUT THE WX GIVEN; IE; RVR'S GOING UP AND DOWN FROM 4000 TO 1200 RVR'S WITH ACFT BREAKING OFF APCHS; REQUESTING WX FOR OTHER RWYS; NUMEROUS VECTORS FOR BRAKE-OFFS; HOLDING PATTERN INSTRUCTIONS; ETC.