There is a bleeder screw on the control valve just like on brakes. Just connect a hose to it routed to a container and then open the bleed screw. Generally, when the cells are bad, there will be little residual presure in the system since the cells no longer hold presure and are now effectively just big oil tanks (they will be full if bad-and heavy). You are still gonna spill some oil. Get a new filter and several bottles of the oil. Do a flush while you are working on the system. Fluid should be replaced every two years or so. Cells are the cheap parts of the system. Everything else costs mega-$.