It sounds like you have original cans, so it should be clean.

If you only have the one leak that you have identified, there is really nothing to gain by flushing and all that. Since you have a new f/d, then putting it on would be a good thing, but you have no reason to flush the system. Simply replace the expansion valve first, putting a little refrigerant oil or nylog on the o-rings, once that is tightened up, then replace the f/d last so that you get minimal moisture into it.

Then using a motorized vacuum pump, evacuate thoroughly, do a gross leak check by seeing that it will hold vacuum for about five minutes then evacuate a little longer. If it is warm weather, you should be able to adequately evacuate in a few hours. If it is cold it will take much longer to boil out the moisture.

The only time you need to thoroughly flush is if you have blown a compressor that scatters debris through the system or if you are converting to a different refrigerant meaning that you must get all the old oil out.

Good luck,