Video shows the four gasket/O-ring configurations used to connect the probes and hoses to the brew and steam boilers on a BES900XL.
1) All 5 probes have the larger blue o-ring and no washers
2) Simplest hose connection uses the standard small orange o-ring only (no washers)
3) Some hose connections have a washer above the small orange o-ring
4) The steam boiler hose connections from-pump and to-brew boiler (the two with boots) have a double orange o-ring and a washer above them (not the same small orange o-ring used on all the other hose connections).