Does your truck have an engine driven fan ? or an electric fan? or an auxillary electric fan to cool the condensor.if it is an electric fan and doesn,t come on at idle it will cause the pressure relief valve to open.As the high side PSI starts to rise, without airflow past the condensor it will just continue to climb up till the pressure relief opens.This can also happen if the high side cutout switch is faulty too,but something has to cause the pressure to rise first.
Did you here any noises that sounded like a pack of firecrackers going off all at once?Thats the sound that the FWD cars would make when the pressure build up would release all at once,but some just make a loud hissing sound.
Also make sure your A/C condensor isn't plugged up with leaves or mud.Also as mentioned before check for any kinks or dents in the smaller of the 2 lines between the compressor and evaporator and either of the 2 lines leading to the condensor in front of radiator.
This ^^ x2.
Something is causing the High Side pressure to skyrocket and blow off the pressure relief. The most common cause is poor/no airflow over the condenser. This is usually caused by a faulty fan clutch or the heat exchanger stack blocked with dirt or debris (or both).
Soak the radiator and condenser with a strong detergent like Simple Green or Purple Power. Let it sit for a few, then blast it thoroughly with your garden hose You'll be amazed at how much crud washes out.
If you can, separate the radiator and condenser, then clean the area in between them. Again, it's amazing how much crap gets caught between the 2 heat exchangers.
The engine is very tolerant of airflow restriction and won't overheat until a fan clutch is nearly dead or the heat exchanger stack is almost completely blocked. The AC system is VERY sensitive to condenser airflow issues, it doesn't take much at all.
Usually, the first indication of a weak fan clutch is poor AC performance at idle.
Being an F550 it "might" have an air or hydraulic fan clutch, or so I recall. I never spent much time working on trucks bigger than F350. If so, make sure the clutch is engaging.