When operating the loader, the 3pt lever should be in neutral.
If it is in the up position, the 3pt would be raised and the fluid is passing over the relief valve, and causing the loader valve to act strange.
By locking the 3pt on, you have pressurized the entire path from pump to the 3pt.
Set the 3pt to neutral or about the middle of the lever range.
If you had a gage, you could see the pressure developed by the hyd's
Since you have not blown anything yet, the fluid from the pump must be going to tank.
The fluid in an open center system should flow in a series path.
The path should be something like this. Pump to hyd block, from hyd block to loader valve, PB from loader back to hyd block,then from hyd block to 3pt. The return line from the hyd block should go to tank.
The PB going back to the hyd block, can be used for a remote valve which would be placed between the hyd block and the 3pt.
Did you do the plumbing ?