You should be able to examine the expansion valve and determine if it is leaking at a connection or at the bulb. I had one fail a few years ago and was leaking freon around the bulb. If that is the case you have to replace the expansion valve. If it is leaking at one of the connectors, then new o-rings with the nylog sealant should fix it.
Since you're planning on peeling the system apart anyway, you might just buy an R134a expansion valve. This is one that replaces the original and is calibrated for 134.
To flush, you would be best served by purchasing a flush cannister and gun from your automotive a/c supply house. You take the top off the cannister and pour in the flush agent. You put the top back on and pressurize the cannister with compressed air through a shraeder valve. There is a hose and blow gun. After disconnecting all the lines, you put the blow gun at the lines, condensor and evaporator. Blow in some flushing agent then blow it out with compressed air.
This is best done on a hot, dry day so that you get as little moisture in the system as possible. MAKE SURE YOU BLOW ALL THE FLUSHING AGENT OUT OF THE SYSTEM. After thoroughly flushing everything and BLOWING ALL THE FLUSHING AGENT OUT OF THE SYSTEM, you then put everything back together with 134 compatible o-rings coated with nylog. You can get an assortment of 134 compatible o-rings and nylog from acsource.com for about $25.
Once you have flushed everything, resealed or replaced your expansion valve, and buttoned it all back up, you need to pull a good vacuum on the system for as long as possible under the warmest conditions possible. If it is not a HOT day, it wouldn't hurt to use a heat lamp on the evaporator and condensor to help boil out moisture.
DID I MENTION THAT YOU NEED TO ENSURE THAT YOU BLOW ALL THE FLUSHING AGENT OUT OF THE SYSTEM?
Make sure you have really looked over the system with your black light to be as sure as possible that this is your only leak.
As you go through the process, let us know if you have any more questions. You would be well served to find a local automotive a/c supply house to get flush cannister, flushing agent(some people use mineral spirits, but they leave a residue) and whatever you might need.