Google search "patents" then "hydraulic drawbars" for ideas
https://www.google.com/patents/US3487448?dq=hydraulic+drawbar&hl=en&sa=X&ei=YToSVZL7K_WKsQSO3YL4DA&ved=0CCoQ6AEwAjgK
https://www.google.com/patents/US2922660?dq=hydraulic+drawbar&hl=en&sa=X&ei=2TgSVf_OE4risATa6oGgDg&ved=0CDQQ6AEwAw