lost all hydraulic pressure and have check all obvious check valves and reliefs cant find anything wrong, all fluid is returning to tank is there a start up pressure to activate pressure reason i ask is from what i understand these pumps create flow not pressure but all is pointing at the pumps. i have also blocked all lines into spool valve and still unable to build pressure that's why i feel the pumps are shot but goes against everything in my brain of what i was tought if need for added info has a john deer 4045t engine and the pumps are a vickers g2020