What was found
FLIGHT 5116 HAD A LOSS OF PRESSURIZATION AT 6000FT OUT OF STL. CAPTAIN STATED THEY LOST THE RIGHT HAND PACK WITH THE LEFT HAND PACK ALREADY DEFERRED. DECLARED AN EMERGENCY AND RETURN TO DEPARTURE. AIRCRAFT IS OUT OF SERVICE. TROUBLESHOT IAW FIM 75-30-00-810-811, FOUND PRSOV FAILED. R & R THE RT PRSOV IAW AMM 36-12-01-000-400-801, OPS AND LEAK CHECKED GOOD. DURING THE TROUBLESHOOTING OPS CHECK AND ENG RUNS AT HIGH POWER, MESSAGES DISPLAYED AGAIN. INTERROGATED MDC, FOUND B1-349539 T/S PER FIM 36-21-03-810-814, FOUND RT PLENUM LEAKING AT MULTIPLE AREAS. REPLACED THE RT PLENUM . OPS AND LEAK CHECKED GOOD.