Early model 30 series had that valve manifold at the compressor. Every one I do A/C work on where I have the lines loose from the compressor, that valve goes bye bye and the later style lines go on. I stock the later lines on my service truck mainly just for that purpose. To answer the question about the position of the valves, turning them in will close off the system. Mid way will allow for flow through the manifold while allowing pressure to the service ports. When you remove the gauges, back the valves all the way out to allow for full flow through the manifold and block the pressure from getting to the service valves where there may be a possible leak. Good luck.

Here's a chart for pressure versus ambient temp. http://www.ackits.com/aacf/ptchart.cfm Notice how much the recommended pressure varies for 134a. In my Opinion the biggest problem with conversions is running too high a pressure to compenstate for the fact that 134a doesn't cool as well as R12. The good thing is that Deere somewhat oversized the system to make up for dirt and air leaks so a conversion will work. First thing is to ignore the sight guage. If you fill the system full enough to clear out the bubbles then the system is too full and you'll eventually find out where the weak points are in your system. I try to run as low pressure on the high side to just get the job done and spend my time keeping the evaporator and condensor as clean as possible.So keep the drain lines for the evaporator clear so that it doesn't get mud forming up there. Make blowing out the condensor part of the front axle greasing. Keep the seal between the evaporator and the top plate in good shape to not allow air to by-pass the evaporator.