A hydraulic tool assembly for a downhole tool comprises a body (12), first and second members (14, 16) mounted for independent movement with respect to the body, and first and second control fluid chambers (24, 26) associated with the respective first and second members. Movement of the first member (14) between a first position and a second position in response to an applied force displaces control fluid from the first chamber (24) into the second chamber (26), to move the second member from a first position towards a second position to execute a tool function.