I am going to offer a different opinion for the plumbing the return line for the post driver. In my experience, the driver works much better when return goes to the pipe plug fitting the original poster showed. When returning to the filter cover, flow is restricted too much and the ram does not drop fast enough to efficiently drive posts. At least in Maryland with Hard clay soil and shale rocks. The small diameter cylinder on the driver does not require a large volume of oil that will starve the pump for oil when returning to sump. At least it won't if closed center valve is used on the driver. Open center valve will not work well at all if returning to sump.