You mentioned that you pressurized the muffler but was curious if the muffler is maintaining what you pressurized it to?Or is it leaking out into the oil?Also when you added the air did you do this with no oil pressure on the line by either buffering the car or using the machine gate valve and then opening manual lowering to releive pressure.I have also seen that if the packing is wiping the piston to dry then you can get some vibrations as a result especially at leveling.I have used every cloth on the piston above and below floor level heights to help the piston trap oil to keep it lubricated and had good results.Not to drag on but,be sure that the discharge hose in the tank is not near the intake of the pump,this can cause some issues with oil getting aerated and behaving similar to what you have.Sorry for the short novel,hope this helps.
