Arrrrghhh, my A/C won't work!!!

I live in an old house with an old a/c unit which hasn't been blowing cold(fan only) for the past week. I finally got the repairman to come out today and he determined that there was no water flowing to the unit. It would shut itself down to keep from overheating. He replaced the water pump($430) and it started blowing cool again. An hour later it turned itself off again and would only run the fan. I called the repairman back and he stopped by within a few hours and then determined that it ran out of freon. He recharged it and didn't charge me a dime since he felt he should have checked it with his first visit. It once again was blowing cool air for the few hours I was there. I left it on when I went to work but when I got home only the fan was running.
Before when I turned the unit on it would come on for a minute before shutting itself back down. Now only the fan is coming on. I'm praying its not something major as I will be unable to have it fixed anytime soon. Any ideas on what the problem might be now?

 

If that is the problem then I'll be extremely happy. Freon recharge should be much cheaper then replacing the compressor. I've delayed my first HDTV purchase until I get this crisis resolved and may cancel it altogether if the repairs become too costly.

 

Update: I got back home from a trip yesterday and had the repairman come back out. You were right Steve, the Freon leaked out because the valve stems were bad. He charged me $102 to replace them and the Freon. Its blowing cool as I type and I'm relieved it wasn't anything major. A new compressor would have put a serious dent in my HDTV upgrade plans. Keep your fingers crossed that nothing else goes wrong and thanks for the help.