The biggest issue with brake fluid contamination is water. Brake fluid absorbs water like crazy. The water is heavier and eventually settles at the lowest point, usually the pistons. This causes rust which wears out the o rings on calliper pistons...probably the main cause of brake piston leaks.
The master cylinder reservoir isn't vented to atmosphere and materials in the braking system are designed not to chemically react with the fluid, so unless you open the cap, brake fluid contamination is very unlikely.
That being said, if the manual says flush the fluid, do it, but if the cap hasn't been off, any contamination came from the truck, not you.
__________________
'00 Quad Cummins 4x4 Shortbox, Silver. From AZ to AB. Finally installed; Adrenaline (impressive) w/ Pulse (less so), Raptor, Dodge Steering Stabilizer, Luke's Link, Leer tonneau. Still to come; Mikuni MX-40 coolant heater, Tires...