Eagle Repair: A/C, plymouth acclaim, 134a

Question
I have a 1994 Plymouth Acclaim.  I realize this isn't an Eagle, but I am hoping that you can help me anyway. The A/C was working fine one day, and then the next day it stopped working.  I bought a kit which contained a 14oz can of 134a which actually contained 11 oz of R134a, 2 oz of oil, 1 oz of maxi-boost (to make the A/C colder) and some leak sealant (which supposedly seals most leaks).  It also came with a cable and a guage to check the pressure.  I hooked up the cable and guage to the low pressure side as explained in the directions and turned the A/C on.  The reading was close to zero.  I then filled up my A/C with the can and the pressure goes between 21 and 24 depending if the compressor comes on.  The instructions said that it should be between 25 and 44.  The A/C seems to be working fine now.  Here are my questions:

1. Is R134a supposed to dissipate over time, or do I have a leak somewhere?  (I bought the car used last summer and never had a problem with it so I don't know if this is normal).

2. They sell the R134a, oil, maxi-boost and leak sealer separately.  How much more of each would you recommed putting in to fill it?

3. If I do have a leak, what are the chances that the leak sealer will fix it?  They sell 3 oz cans of leak sealer.  Should I put this in?

4. If I do have a leak what would it cost to have the compressor replaced? Condenser? O-rings?

5. I was just curious if R12 is the same as freon?

6. This question is from my friend. He filled his car up with the 134a, and his car stays cool for a few minutes, then it isn't cool anymore.  He said that it seems like the compressor shuts off.  If he tries it again later on, it will be cool again.  Is this a compressor problem or something else?

I really appreciate your time in answering all of these questions.  Thank you very much!

Answer
I'm not an AC guru (I just have the basics down, and I learn each time I talk to someone about it just how little I know)
  The leak that drained it in the first place may be why the pressure isn't coming up, or the low pressure sensor (on the line with 2 wires going to it) is cutting out too soon. I will one day take a course in AC and I'll know more.