Just a small addendum to the above topic for anyone completing valve core removal...just completed core and O ring replacement of original which was still in car...
1. Punch it down a "short" distance to remove retaining clip i.e. just enough to slide the clip out.
2. Use a magnet to hold arm, clips, screws and washers from falling into cavity..and use it to recover them if they do fall down. 3. Old O ring appears to be square cross section whereas new one was round cross section.
4. Spray small quantity of Houdini lock smith oil which penetrates, cleans and lubricates with suspended graphite...makes removal easier.
5. If you do push either new core or old core too far into body, the ring will catch on inlet port when you attempt to remove and appear to be stuck.
6. Take a very thin plug gap wafer and slide down to prevent o ring from catching and slide up and out..6.Using magnet put everything back in place exactly where it was before dismantling....
Works perfectly and now have heat in cabin with slide that opens and closes easily..
Ten minute job if you exclude the 2 hours or so looking for plug gap wafer, dropped screws and washers, scratching head, looking up tech manual for process and going to store for another O ring damaged during learning process..