[60% propane / 40% isobutane works better.. pressures are correct.. but
still illegal in the US under current EPA rules]

>While we are on the subject, has anybody found a good source for 
>isobutane?  i've been buying the MSR stove fuel at a camping store for 
>$3.00 / 6 oz can.  works ok and i only use 2 cans for a complete fill up 
>but it should be a lot cheaper if i could buy it in bulk somewhere like 
>propane.


Here are some bits of information that we learned at Osoyoos: (5/5/5)
1 AC technicians do not like to use 134, it is harmful to their health.
2 You do not need a vacuum pump to charge with Hydrocarbon refrigerants like DuraCool (Propane).
3 In fact they work better if you do not pull a vacuum.
4 Propane will mix with whatever you have in your system. There’s nothing to change.
5 Recover the existing refrigerant (most of our systems are already empty).
6 Add 3 cans of DuraCool
7 Use Glacier Gold if you have a leak, every can has a dye for leak detection.
8 Run the compressor, and measure the low side with your tire gauge.
9 You should have 30 lb of pressure. If it’s too high let some out, a little low will be ok.
10 Put on the 134 adapter connections, and the system is legal.
11 Mark the system as filled with DuraCool or Propane.
12 You are done, no gages, no pump, no changes.
13 Propane is more efficient. Your compressor will use 40% less horsepower.
14 The head pressure is lower with Propane, the compressor will last longer.
15 It will cost you $30 for the Propane and taper hose.
16 Propane has a higher auto ignition temperature than 134. (Propane still lights more easily)
17 Propane leaks less than R12.
It has never been easier to do your own AC work. It is interesting to note that there is also a hydrocarbon mixture that works on a home or coach AC to replace the R-22 with the more efficient refrigerant.
Note: The EPA has issues with Hydrocarbon refrigerants. You should read this link and make your own decision: http://www.epa.gov/ozone/snap/refrigerants/hc-12a.html#q9
The above information and more technical detail are available on these web pages:

http://glacier-gold-refrigerants.com/index.html
http://duracoolky.com/refpage.html
http://www.duracoolusa.com/
http://www.duracool.com/