Bruce
At twenty years there are a few things that can help to extend it's life. In no particular order:
Oil all motors - 3 drops for each bearing - repeat yearly
Oil the squirrel cage bearings and replace the drive belt if equipped - repeat yearly, or as needed with the belt.
A "Zoom Spout Oiler" is easy to use & has the correct oil
Clean fan & squirrel cage. Strong degreaser & stiff brush. Getting each vane of the squirrel cage clean is important.
Carefully clean both coils, then buy an electrostatic filter & wash it every month that the AC is in operation.
Replace all motor capacitors - once every ten years.
Capacitance is printed on each piece, most look something like a short roll of nickels
Install a "hard start kit" for compressor - once every ten years
You'll need the compressor make & model
Consists of a Start Capacitor, Run Capacitor & relay & should come with instructions
Pull the breaker before working on the unit. Short out the compressor capacitors,
they bite.
A 20F split is what you want between the return air & a vent, if it's lower your charge may be low, if it's higher your evap coil & or fan are dirty (or you've a very old, very dirty filter)
If an HVAC supplier won't sell to you try Grangers, their catalogue has everything you need.
It's cheaper to replace the capacitors than to buy a quality tool to check them.
A cheap "fin comb" can straighten crushed coils.
If you've a decent Amp-Probe you can check your compressor for excessive draw, but the hard start kit will probably keep it running properly for years.
If your system hasn't leaked in twenty years it's not likely to start now.
I've seen 40+ year old systems operating in Las Vegas heat that ran perfectly. Capacitors do break down after extended use & when they go they take the motor with them - same with the larger capacitors on the compressor.
The above probably won't make your unit work better, just longer.
Graingers used to carry evap. coolers, though I don't recall the brand. Hard water requires that you drain a portion of it more or less constantly. A "Tee" fitting off the pump is the usual solution. When the pads get crapped up they need to be replaced, but they should last for one season.
You don't need to wait for late spring to set up your AC.
Have Funn!!
Terry