Yes, the return oil must dump quickly.

 Dad's 4020 with Shaver post driver comes either 1 or 1.5" return hose and goes into a welded adapter, right into the tranny oil filler hole, under seat.

Edit: Here's an old picture, I drew for a previous post:

Here's a photo of how small diameter of hydraulic cylinder, on a Shaver post driver = doesn't need that much volume.