just a lil piece of dirt or particle will clog the expansion valve. i honestly would change it. take some lines apart get a air hose and just blow them out. the compression fitting was venting due to the high side being to high. Its a pressure relief valve. Don't worry it was just doing its job. How long ago did you replace the expansion valve? Was it a new one? IF old it might be a r12 expansion valve. At any event get a new expansion valve try cleaning the lines out. Pull a vacuum.. and no pulling a vacuum will not tell you if you have stuff in the system. You have to make sure you don't by cleaning out all the lines and making sure. Id clean the lines all the way to the compressor.