A working vehicle has a castor wheel assembly selectively providing steering assist. A pivoting caster spindle coupled with a caster wheel and an engagement plate connected to the caster spindle in a fixed manner such that the engagement plate pivots with the caster spindle. A pivoting steering arm is rotatably connected to the caster spindle and a steering cylinder is connected to an outward end of the steering arm with a tie rod configured to pivot the steering arm. A keyed connecting mechanism selectively couples the steering arm with the engagement plate. In an engaged position, the keyed connecting mechanism couples the steering arm with the engagement plate such that the steering cylinder pivots the caster spindle and caster wheel. In a disengaged position, the steering arm is uncoupled with the engagement plate such that the caster spindle and caster wheel are free to pivot independent of the steering cylinder.