I have seen people struggle with pushing back in the C-130 family.
Even though there is no PushBack button, it is very simple.
You just have to disactivate brakes and do the same thing as reverse thrust (pulling throttle to -100%) and voilá, you should be pushing back.
Note: if you are flying too fast in the C-130, you can also reverse thrust in mid-air!
I hope this helped!