An apparatus for releasably holding a tubular toolholder shank includes a tool support member; a canister disposed within the tool support member; a lock rod movable within the tool support member in a reciprocating motion for pulling the tubular toolholder shank into a locked position and for releasing the tubular toolholder shank to an unlocked position; and at least two locking elements engaging the lock rod so that movement of the lock rod causes the locking elements to extend outward to engage the tubular toolholder shank. The lock rod includes a relief cavity extending rearward from a forward end of the lock rod. The lock rod has a maximum outer diameter and the relief cavity has a maximum diameter. A ratio of the maximum diameter of the relief cavity to the maximum outer diameter of the front portion of the lock rod is greater than approximately 1 to 4.