Auto Air Conditioning & Heater Repair: Auto AC not as cold as it used to be, gmc suburban, record temps

Question
I have 2 vehicles that both have the same problem. As they are getting older, the AC is not as cold as it used to be. Does the freon dissipate in any way through time? If the freon is low, is there a leak or is there an explanation of what happens to the freon?  

Vehicle 1 is a 2004 Crown Victoria and vehicle 2 is a 1998 GMC Suburban. I bought both used and the AC used to freeze you out, now it will keep you cool but not 100% comfortable. I usually work on my own cars and I am pretty good with the actual repairs, but not the diagnosis and I am not sure that I want to try to diagnose this myself or take it to a shop. I am trying to avoid a large repair bill and I would like to get some information before deciding on going to the shop with the vehicles or doing it myself. I live in Florida and the AC is used most of the year.

I would appreciate any information that you can provide.
Thank You in Advance!
Rick

Answer
Hi Rick

1 freon does not dissipate or gets consumed.

2 Usually if it is low you have a leak or someone has pulled the freon out.

You need to get the pressures for me both hi and low. Remember we have had record temps. this year so you have to allow system to do it job under extreme conditions.