Case 580B hydraulic/front loader issue

Let me start by saying this 1970's era machine sat for an unknown number of years (guessing 10 or so), I have owned it for 15 months as of this post date. Total basket case, engine was seized (a story of its own) and numerous other problems.

It is a playtoy project and so far I have rebuilt the engine, replaced needed hoses, all fluids and filters changed throughout, have 4 of the BH cylinders at the shop for repacking (all glands frozen), and a lot of other repairs.

The issue I am having is.....I disconnected the 2 quick connects to the BH due to the cylinders being removed, I filled the hydraulic reservoir for the first time and started the machine. At this point I was going to try purging air out of the system by rolling the bucket forward (all cylinders were drained by us, so we could lift the bucket and secure it when we brought it home), as soon as I stopped bucket movement the engine bogged down and stalled out.

As long as I am rolling the bucket one way or the other, it is fine, when I release the handle back to center the pump loads up and it will stall the engine. There is no issue for the loader lift circuit, only the bucket roll. Does this have anything to do with the BH being disconnected from the hydraulic circuit and not returning oil?

Did you join the two backhoe hoses together after you disconnected them from the hoe,you must otherwise the oil that would be going both in and out of the hoe control valve is dead ending,ie the loop is blocked,if you did connect them with the engine stopped disconnect them again,push the plunger valve in each coupling to let the pressure off in the hose and connect them again making sure they go home and try that.
AJ