The fault you have is not in the unloader & flow control valve as it is still there when u pull the aux valve lever to dead end the oil supply. At that point oil can only go 2 ways, to hitch cylinder to lift if draft valve is in lift mode , and to blow the main R/v and pass to return.
There is a problem with the linkages in the hydraulic top cover, it may be a circlip off or more likely the sensing beam and spring have become detached from their mounting pegs (hook end of spring may have broken off !! and as such the walking linkage has gone out of balance !!
To check, just rear of unloader &flow assy is a metal plate with the breather on it, remove this and you can see the linkage that operates the draft valve, operate the draft lever from full lower to full raise & back to lower again. (Engine stopped !!) and you should see the spring expand and contract. if it does then its a circlip issue, if it doesn't its a broken or detached spring.
Some skilled mechanics can replace spring through this plate but its very fiddly. Official way is hydraulic top off, replace, bleed brakes etc, allow about 12 hours labour.
I need your e mail so i can send document that shows the linkages!!
Also do you have a service manual ??