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I believe the hydraulics are open centred. If so, I'm assuming that the pressure relief valve is not opening when the tractor is running. I wonder if the 3ph & scraper ever worked normally after the loader was installed. I also wonder if the scraper hydraulics run off a control valve that's on the loader SCV or has it's own pre-existing valve. I finally wonder if the loader SCV is a power beyond valve (two hoses from the outlet section rather than one). This info would help diagnosing the problem. I suspect it's a plumbing issue. For example, many tractors power external hydraulics from an auxiliary hydraulic system. There is a diverter valve that selects between auxiliary and remote system use. The remote system powers only the 3ph, while the auxiliary system feed oil to an external port. When using the auxiliary system a return line to the sump has to be provided. Many auxiliary systems are set up so the auxiliary system return feeds into the 3ph high pressure line so the a loader and the 3ph can be used alternatively without changing the diverter valve. In such a system, the hitch will not operate when any of the auxiliary hydraulics are used. In an open centred system, the hydraulic pump circulates oil continuously through the system. When a valve is operated flow line is closed and lines to a cylinder are opened. Pressure is developed because the oil flow to the sump is blocked, and oil will flow to a cylinder causing it to move. The various control valves (including the 3ph)are hooked into the line in series-oil from the pump goes through the first, to the second etc. and back to the sump. The main point here is that if several valves are operated at the same time, only the first valve in the line gets pressure. That's why a 3ph stops when the loader is operated. Of course, it is possible to plumb systems in other ways, but I believe what I described is the most common. In addition, the presence of power-beyond, or lack of, complicates the issue a bit. What I'd do is to poke around in the system enough to identify the various components and determine if the system is plumbed generally as I described. Then, I'd ensure the diverter valve is fully in the auxiliary system position. Finally, I ensure all loader and scraper control valve are centred and that the pressure relief valve isn't opening. I'd actually jiggle the valves to ensure they are centred. I have had an intermittent 3ph that cleared up after I jiggled the loader valves. I'll note that that hasn't happened since I changed the hydraulic oil and filter. If that doesn't clear the problem, I'd ensure any power beyond connection are correct. If the problem persists, I'd run a pressure test at the 3ph or separate scraper valve--which ever comes first.
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