A hydraulic sectional control valve includes separate pump pressure and function control spools. The spools may be actuated independently and sequentially to provide raising, holding and power down functions for earthmoving and construction vehicles. Additional function control spools may be added in...http://www.google.co.uk/patents/US4570672?utm_source=gb-gplus-sharePatent US4570672 - Hydraulic control valve with independently operable bypass valve
