What was found
DURING CLIMB, THE RIGHT PRECOOLER FAIL LIGHT ILLUMINATED. THE CREW AND PASSENGERS EXPERIENCED DIFFICULTY BREATHING. THE RIGHT BLEED AIR SOURCE WAS SELECTED OFF AND THE AIRCRAFT RETURNED TO DEPARTURE AND LANDED WITHOUT INCIDENT. TROUBLESHOOTING INCLUDED SWAPPING THE FLOW CONTROL VALVES FROM LEFT TO RIGHT AND RIGHT TO LEFT. ON A SUBSEQUENT FLIGHT THE LEFT PRECOOL FAIL LIGHT ILLUMINATED. FLOW CONTROL VALVE WAS REMOVED AND AN OVERHAULED FLOW CONTROL VALVE WAS INSTALL. NO FURTHER PRECOOL FAILURES HAVE OCCURRED DURING APPROXIMATELY 10 FLIGHTS SINCE THE CHANGE.