I have a 86 4-Runner with factory A/C. It came with R-12. I would like to replace it with Freeze-12 (it's 1/3 cheaper and you don't need a license to purchase). Can anyone tell me approximately how much R-12 my system would hold if it was completely empty. The Freeze-12 requires about 10% less than R-12.

Don't do it. Spend the money and get the R-12. Anything else is garbage. Either it is dangerous, will ruin your r-12 system, or will not work well. For a few bucks you can get certified at http://www.macsw.org/
open book test.

first, why are you converting? leak? system not cooling? you need to solve those problems first. i just converted my system to R-134a, and it was pretty simple. i looked at the freeze 12 stuff, its 90% 134a and 10% some other refrigerant, something like R-142b. mixing refrigerants isnt a good idea, as they have different weights and will circulate differently, and the 134 wont circulate the oil. it costs about 30 bucks for a new reciever/drier, and if you have a leak, 10 bucks for the o-rings. 35 i paid for the conversion kit. change the oil to ester oil and you should be fine. mine works great, i get about a 25 degree drop in temps. i wouldnt keep the r-12. its the best refrigerant ever made, however you need a license to get it as stated above, you have to get it from an HVAC supplier, and its EXPENSIVE. youll also need a manifold gauge set that connects to r-12 systems, the only place that sells em is HVAC suppliers or ackits.com. id go with the 134a conversion if i were you. my 2 cents.

Your system should hold about 1.8lbs R-12.

If you have substantial repairs to do, go ahead and make the complete conversion. They alternate charges like freeze 12 imo are going to give you probs down the road. The oils to lube the system are completely non compatable though some will swear they never had a problem with Ester 100. If it's a simple matter of an oring or hose then I just stay with the freon and go have it evacuated real good and recharged . A good condition system without leaks should go 5 or more years without further maintenance

I did the R-134 conversion(evacuated and added 134 oil/refrig. w/r-12 comp.) 9yrs ago when my system needed recharging at $70/lb, I was never that pleased with it here in the CA sun. It would cool things down, but never made it anything more than liveable on 106+ day. I was never able to run teh a'c on low like I was able to do with the r-12. My comp was starting to make noise about 2yrs ago, so I looked for an alternative. I cane across a product called MaxiFrig, look here for a Maxi-Frig vs R-12. I am on my 3rd summer with the original same 2 cans I put in, no leaks, and no need to recharge. I don't have to run the a/c on hi, just to get cool enough to be tolerable, I have to turn it down to low because it will get too cold. I am not concerned with it being propane based, it isn't pure propane so it shouldn't be that volatile. Even if you ruptured an A/C line in an accident there isn't that much in the system to start a fire, more likely to start a fire from teh ruptured gas line and the exhaust manifold. The main concern would be a leak into the cab, though with any refrigerant a leak into the cab would be a concern, even more so with the 134 IMO.

Which ever way you go you need to determine why the current system is dry and fix it before adding any new refrigerant.

My system is low on R12. DO I have to have the system evacuated and a vacuum pulled to remove the R12, or can I mix it with the maxifrig?

Adam . I doubt you can mix it. Take your vehicle to a a/c service center and have them reclaim the R-12 . You want to remove any existing freon slowly so it doesn't carry the oil out with it . Maybe a shop will do it for free since your giving them the freon? Pulling a vacuum is needed to remove any moisture that will freeze up and clog internal components till it thaws again. If you install the Maxfrig , let us know how it performs . Specs advertized sound almost too good to be true. I'd like to try it myself , but I imagine most shops are going to try to talk you out of it

A note on Duracool, HC-12a etc...
Reading this got my interest, so I did some Googling.
Came across this site:
http://www.epa.gov/ozone/snap/refrigerants/hc-12a.html

If you can wade through all the technical/legal stuff - basically it says it is not legal to replace R-12 (e.g. "freon") with Duracool. It IS, however, legal to use it as a replacement for 134a.

According to the paperwork sent with my order, Maxifrig is compatable w/both R-12 and R-135 oil, not sure about mixing the refrigerants. When I converted from r-12 to 134 I had a shop evac. the R-12, when replacing the R-134 I had a buddy's shop remove the r-134 refrigerant. The first place I went to said they wouldn't touch it because it was an R-12 system, something about contaminants from the R-12. If you want to try the Maxi-frig and don't want to buy a whole case, let me know, I have 16 cans left. I have convereted a couple more rigs this spring.

If the shop does evacuate the entire system, you can get a can of R-134 oil, you will also need to get the R-134 conversion nipples, and the hoses/gauges. You can't get the R-12 equipment anymore(atleast not here in CA) w/o a license, and the older style R-12 hoses don't fit the Maxi-frig cans. It might be cheaper to buy the R-134 conversion kit over buying the pieces individually.

My 91 Montero last year started leaking out of the viewing glass on the dryer. Only thing that was ever done to the AC system was putting more R12 in it about 5 years ago. So a fewmonths ago, I bought a new receiver/dryer, installed it, and evacuated the system. I filled back up with 2 cans of Freeze 12, $7/can at Napa. It blows cold and keeps it cold in my black truck here in the 100+ degree SC weather this week. Unless I can get real R12 for $10 or less per can, I will use Freeze12, works great.