Remove the front drivers side tire, then try locating the low pressure valve (about 11 o clock when looking at the axle) under the engine on driver side. Theres a panel covering it, remove and you'll see it, normally they have blue, or black valves but this one is grey either way e-z cool or any other store bought should work. just watch the psi and stay under 40 to b safe. good luck im not a professional but this is how i did it.
the compressor is located in the lower front drivers side wheelwell. This is also where you recharge your A/C.
the low end pressure servvice port on a 3.9 litre 2001 Lincoln ls is on the ac compresser wich is located on the lower drivers side of the motor. easiest access is to take the drivers side tire off under the a arm.
You can recharge your 2004 Lincoln LS from the low pressure port. The low pressure port can be found on the top of the air conditioning compressor.
turn the car on
Service port is on compressor under car on drivers side, remove plastic scuff plate and you will see it.
Check relay
$23
40 psi
I'm far from an expert but it sounds like you have a small leak or had one and the A/c just needs a recharge.
Down on AC compressor
AC filter change required
The hookup is on the passenger side by the firewall, silver canister.