Originally Posted by NZConvertible

You better believe they do! ALL A/C systems will eventually lose enough of their refrigerant to degrade preformance. A car system should last about 10 years if nothing fails and causes a big leak. When the sytem is recharged it will be both evacuated abd pressurised to check for any leaks. You will generally need a new dryer if it's the original one.
To check the if you need a regas, turn the A/C on and you watch the sight glass (infront of the radiator, little hole in plastic cover). If you see bubbles you need a regas.
The biggest problem is likely to be the type of refrigerant currently in the car. A lot of cars this age still have the now banned R12 in them. It's illegal to refill the system with R12 (if you can even find it), so it must be replaced with a more modern CFC-free refrigerant, usually R22. The problem is this stuff is not compatible with the mineral oil in the compressor. All the old oil must be flushed out and replaced with synthetic oil. Ring around a few automotive A/C places and get quotes.
Just rechecked the original question. When you say blowing hot air all the time, do you mean even with the A/C off? Might be a Logicon problem. Whole different kettle of fish!