A pump with the O ring on the piston only gets away with it if the pressure is low. When pumping, both the motion and the air pressure tends to roll the O ring into the gap between the piston and the cylinder wall. If you are air actuating a piston, then O rings on it are correct. If you are pumping, you want the O ring in a groove in the cylinder and the good finish has to be on the piston. If you need to accommodate a taper, a lip seal may be a better choice. Since you don't say which way it is going to be, this dissertation may be unneeded.
Bill
P. S. If you do have such a mold made, remember that salt in perspiration rusts steel and resist the temptation to run a finger over the polished surface. If you do, do not expect understanding from the mold polisher. You will not get any. I have made a few polished molds and it is tedious, demanding work.
