The 400 has an adjustment on top of the control valves behind the dash. The selector changes the function of the valve between single & dual acting. On a well used tractor, they're usually hiding under dirt & oil on top of the valve block(s). After you clean the valves, you'll see the selector I'm speaking of. You can use a 1/2" wrench to turn the selector, it moves in an arc from about 10 to 2 o'clock, when looking over the top of the valves toward the seat. There should also be a "S" & "D" in the casting the selector pokes out of. Also, wiggle the handle you've been using to figure out which valve to adjust, if you have more than one valve. Right, center & left levers are not connected to the valves in the same order, nor are the hydraulic lines from them. Anyway, once the valve is changed to single acting, the hydraulic fluid should flow back out of the cylinder to let the Bush Hog down. If your hooking into the hydraulic block on the side of the battery box, I think the hose goes in the bottom connector.
Hope this helps.