The ester oil does not mix with the old mineral oil. Hence you need to remove the compressor to discard the old oil. Some compressors have a drain in the back, but best to remove so you can turn/shake etc to get it out. Add a bit of ester oil in there, then pour again. Best to get it all out. Realize that there is oil everywhere, and this is why A/C techs suggest a "flush". Oil also tends to reside at the bottom of the condesner - it can just stick around in there after the new system is running, though it will take some space and affect condenser performance.

It probably would have been a good idea to find the source of your leak and THEN install the new dryer. now you are exposing the new dryter to moist air, basically killing it's ability to dry. The suggestion here would be to get cracking at this fix, so you can seal up the system and vacuum out the moisture ASAP.

All in all there are many ways to get the system to "work" and there are more proper ways than others. The o-ring replacement is not necessary at a junction where you are not to replace the components and there is no leak. Basically if you were to completely redo the hole system, you replace all o-rings. If you touch any junction, then you install a new o-ring. Anything more is fine but not necessary.

The vacumming is done by an A/C vacuum pump. I got mine off ebay for 100$. Good investment. Also get a good set of manifold guages, around 70$. Paying a shop to do this twice will pay for the tools for you to keep. For me, with 4 volvos in the family, having this stuff is very valuable.

Typically from what I here the high pressure hose if very often the culprit as the first leak source on these cars. of course though it could also be the condenser and the compressor itself, or at least those will go i nthe next couple years.

Vacuuming will not suck out the oil. It only pulls a vacuum to the closest possible to 29.99 millibars mercury, this allowing for all traces of moisture to boil over and get sucked out. It serves to "dry" the system, and also to provide a vacuum for the refrigerant to get sucked into. If you do not do this your system will deteriorate and likely blow leaks very quickly, and maybe even freeze up and block during operation.

I suggest you get a shop do a leak down test after you put everything back together and you have the new oil installed. Ask them to find the main source of the leak, and then you can take over and do the repair. Realize though that you will need to pay them again to pull the vacuum before charging up the system. This is why it is good to aquire tools.

Good luck,
Greg Mustang
Montreal - Ottawa
Canada