What was found
ON DESCENT, RECEIVED CABIN ALTITUDE LIGHT. DESCENDED THROUGH FL400, DECLARED EMERGENCY. DROPPED O2 MASKS. ON FINAL APPROACH, RECEIVED A POWER BRAKE LOW PRESSURE LIGHT. ON GRND, USED EMERGENCY BRAKES. MX UNABLE TO DUPLICATE, BLED BRAKES & OPS CHECKED GOOD. MX TROUBLESHOT IAW MM & GROUND RAN ACFT. OPS CHECKED PRESSURIZATION SYS & NO FAULTS FOUND. PERFORMED OPS CHECK OF FLT & PRESSURIZATION SYS, WORKED PROPERLY BUT POWER BRAKE LOW INDICATION STILL PRESENT & AN OUTFLOW BANGING NOISE FROM REAR OF ACFT & BRAKES SMOKING. PRESSURIZATION TROUBLESHOT TO PICCOLO TUBE. TUBE REPLACED & SYS OPS CHECKED GOOD. BRAKE PROBLEM TROUBLESHOT TO A FAULTY POWER BRAKE LOW PRESSURE SWITCH.