HI,
From reading your posts I can see you're fairly new to automotive A/C work and very likely these old Mopar systems. So, here are a few things to consider. If you have no idea what is in the present system it would be a good idea to take it to an A/C shop or borrow a refrigerant identifier and see what is in the system. While it looks like the old R-12 fittings are still in place and not R-134A adapters that's no assurance of what some one may have put in there. There are many internet sites that will give you some basic ideas on automotive A/C systems and how they operate and reading the shop manual is a good place to start. If you do convert to R-134A stay away from those cheap/quick conversion kits out there. A/C professionals refer to them as "Death Kits" as they can quickly lead to system and compressor damage.
To convert to R-134A you will need to use the proper oil in the compressor, flush out the oil R-12 mineral type oil and refill with an R-134A compatible oil. I like to use an ester based oil with is compatible with both R-12 and R-134A. You will also need to change the receiver /drier to and R-134A compatible type. All new receiver/driers today are both R-12 and R-134 A compatible. If you don't do this the R-134A will bust the old desiccant bag in the drier and spread desiccant through out the system and you'll have a real mess. 15 inches of vacuum is no where low enough and some thing is not right some where. A good tight system will easily get down to 28 inches and hold that vacuum for many hours. You are not anywhere near being able to recharge your system and get good results when done. Good A/C work requires attention to detail, it's not hard to do but short cuts will only get you in expensive trouble.
