sounds like the power cap has lost its charge. this is the part of the valve that is mounted on top with a capilary tube on it. the other end is a bulb and should be clamped to the suction line after the evaporator. this provides feeback to the valve to meter the right amount of refrigerant. it is charged with a chemical with the same properties as the freon in the system. it is a closed loop of pressure and if it is ruptured, the valve will shut down . the compressor continues to suck in vapor, but with no feed into the evaporator, the evaporator and suction line are evacuated by the compressor into a vacuum. i don't really know if the power cap is removable on that model, but if it is, replacing it will probably fix the problem.
did your tech install a brand new valve ? if so, he should know this is the problem and the part should be under warranty. sounds like you have a long term relationship with this service man. ask him why he wants to convert it. if you totally trusted his judgment, you wouldn't be asking us. was the new problem discovered by him at the time of installation or did it cool and then quit cooling ? if the latter, then the capilary was probably touching something with vibration causing it to rub a hole.
the first set of symptoms were not the same as the second set. justy curious...when both pressures were low, did he attempt to add freon to the system ? what were the results. you seem to have the technical data from his efforts. did both the pressures come up ?