4 Wheel Drive/SUVs: air conditioning, metal shavings, lincoln navigator

Question
First of all, thanks for any help you can provide!!

We had our SUV  (2001 Lincoln Navigator) in the shop over the last 2 years replacing the heater core 2-3 times.
This January '07 they replaced the radiator - it was leaking.  The first time we tried the A/C this year when it was hot, the A/C did not work.
We took it in expecting the shop that did the work to cover any problem.
Boy were we wrong!  They are telling us that the compressor seal is bad and we will need a new one ($700 total expense).
I feel that it is possible that it was something they might have done during the previous visits to the shop.
The service manager is telling me that there is no way they could have done anything wrong.
What are the possible causes of a compressor leaking at the seal?  If they forgot to charge the system up or could overcharging cause this?

Thanks again, Steve

Answer
well if the comp. was changed the you always have to change the orifice which is the filter and also the shinny little round canister on the fender well and the lines have to be flushed.

If the comp was changed there there could be metal shavings in the lines and such that could cause it and then simply age and wear and tear and heat will do it.

Thanks