Yes it indeed does sound like there could be a valve or quick connect problem. Does everything run fine with the quick connects made up to bypass the loader valve(pump to quick connect to diverter valve)? does the steering and 3PH operate with the loader valve in circuit but before you try and work the loader that kills the engine? WHo is the valve made by?
In a open center system, flow is never restricted completely. The pressure may rise when performing work, but the work being performed(lifting a load of gravel with the loader) results in nearly the same ammount of fluid being returned to the outlet of the loader valve from the other side of the cylinders eing operated. If for some reason, the load is excessive, the saftey on the loader valve should bypass the working cylinders and send the flow to the outlet port and the rest of the system. It sounds like you are able to perform some work. Maybe to the point that the other side of the cylinder stops moving air(new system, not fully purged) and you atart trying to move fluid. It almost sounds like the outlet port or the quick connect that passes fluid on to the rest of the system is blocked.
If the loader valve is of the proper type(open center), I would confirm that the pump outlet is plummed to the proper inlet port on the valve(inlet port is usually located near the relief valve). I would also disconnect all the lines from the inlet, outlet and working ports on the loader valve and inspect the ports for any unusual objects(nuts/bolts/washers).
Inspect the quick connect fittings throughly, paying particular attention to the line from the loader valve outlet port. Remove both the male and female parts from the lines, connect these 2 parts together and insure that there is unrestricted flow through them.
