Originally posted by Berkel:
...and I'm not sure how to check the low pressure valve. Just take the cap off and try to push the valve down?



No you don't check the charge of the system that way.
The low side is the one closer to the battery. You really need to use a set of A/C manifold pressure gauges to get a proper reading. Adding 134A will get it to work but for how long?? You have to fix the leak first.
You have a leak some place, one place to check, could be the accumulator which is down below the battery. It could have small pin hole rust spots, from the insulation wrapped around the tank, that causes it to leak.
I found the site, read up on things, at least you'll have a better understanding of whats going on with the system.

http://www.autoacforum.com/categories.cfm?catid=2