I would definately get the 134 kit and until I bought my 745 and lived in the South I had never done any AC work. OK I'm A hillbilly and my wife claims I cobble things but they work (I was a professional mechanic for 30 years)
First of all where did the refrigerant go? Check all the connections, not for tightness but for oil smears. Especially check the condensor since it's not well protected from rock damage, I have another "cobble" job for rock protection. Now that you've changed all the O-rings and replaced or repaired any other leaks make sure the compressor isn't tied up. You should be able to turn the clutch, not the pulley and it should turn without too much resistance or funny noises. OK, don't laugh, hook up a Mity-vac hand vacuum pump to the system, this might require a trip to NAPA or Carquest for some appropriate fittings. With the new kit the accumulator will have a fresh supply of dessicant to soak up small amounts of moisture so just start pumping the Mity-vac for about 20-30 minutes until it pulls about 21" HG. My 87 will hold about 3-12oz cans of 134 and I would check to make sure that your 90's capacity is similar. 134 is sensitve to being overcharged. Don't lose that vacuum as it will help suck that first can into the system. Jump your electric fan to help keep the condensor cool while you're charging and if you maintained that vacuum you may not have to jumper the pressure switch. Next I put the cans in warm water as I'm running the engine at about 1500-2000. You'll have to change the water after each can, the AC guys I knew all used the hot water trick and it seems to make the job go faster. I ended up with 31 degrees at the center outlet. As always, this worked for me but your results may differ and I will in no way be responsible for the success or failure of your repair.
Tom

Went ahead and did the conversion from R12 to R134 on my 87 765T this past weekend. Bought the retrofit kit from the local Volvo dealer(Part Number - 9145660-8) . I also needed the secondary kit (Part Number - 9134032-3). The rep at the counter said that this is a regular stocking item.

It took less than an hour to complete, no surprises.

I followed the directions from http://home.austin.rr.com/davurban/AC_RETROFIT/Page_1.html since the kit does not come with instructions. The kit cost $60.90 and the supplementary kit cost $33.85. This bets the $600.00 that a buddy of mine paid for the conversion on his 86 765T.

worst problem is getting R12...it's about $900USD for a 20lb container...the only size you can buy without an epa license. And even then a lot of jobbers won't sell you the 20lb tank (looks like propane tank).

Go to an online site and take the course/test and get the certificate. The certificate will then allow you to buy some R12 in 16oz cans and you can add some to your system.

Barring that, take the car to a service facility and have it done right. I wouldn't be surprised if the 12 year old system needs a new expansion valve filter and purging/recharging.

If you still have system pressure, you just need gauges (R 12 or the R134a type) if you want to do it right, or you can just add refrigerant until it blows cold, with a simple can tap and a hook up to the fittings, probably on the compressor. You have to know the capacity of the system, like 1200 gms (28 gms/oz) , and keep track of addition but you can do it cheaply. If the system is open, you will need a vacuum pump to dry it out. Still, this stuff pays off in a few seasons or with a few cars to do.