Try & design your truck sets to travel straight on & off stage, with fixed castors mounted in the direction of travel. This will save a lot of drama for the Mechs/Techs to handle. Use castors with sealed bearing axles on the wheels, not the cheap ones that only have a bolt thru the wheel. Check them under load or see there is no side slop movement, while holding the mounting plate in your hand & moving the wheel from side to side, there should be no movement or side slop on the axle.

You can use door stops as a brake, the type that are pressed down with the foot, on the spigot & released by pressing the locking plate around the springed spigot. There are other types using a paint brush, which swivels down & jams against the stage or wedges as suggested.

I find that a wooden braced T piece, hinged to the plynth of the truck, at the cross bar & lies flat on the stage. Is better for pushing & pulling the truck. The T cross piece when it is in the down position, can also act as a brake, that jams the wheel of the castor. You will find it easier to control moving the truck.