Got a 3 ton ruud unit that won't allow us to charge up properly. The lowside pressure drops to zero psig and the highside goes up to 150 or so. A few seconds later the internal pressure relief valve is heard and the compressor begins to heat up. We have changed both liquid and suction king valves, replaced the filter drier, cut the liquid line at the evp. and blew nitro through the coil and the liquid line it self. We also blew out the metering device. When we did the metering device there was lots of oil. When blowing the system the pressures quickly elevated. This tells me there is no big restriction? Does anyone have any suggestions?