this invention relates to braking mechanism for a downhole tool such as a steering tool . the braking mechanism comprises : a body adapted for mounting upon the tool ; at least one braking member which is movably mounted upon the body ; a resilient biasing means biasing the braking member away from the body ; and a damping mechanism connected to the braking member , the damping mechanism providing a damping force opposing movements of the braking member relative to the body , the damping force being dependent upon the rate of movement of the braking member .