I replaced the expansion valve and dryer assembly. Vent temps are 48f to 52f. (r12 system ) I think the valve is a non equalizing type, so it will take a while to equalize. (mine does). Is it possible that your high side gauge won't go higher than 150psi? (stuck maybe) Did your low side pressure rise after you added r12? Never trust a sight glass. I have seen perfect systems running with a few bubbles going through. (flash gas) Avoid the r134 if you can. good luck.