Do I need AC charge?

[paulmars]

What else could be wrong?

10-12° difference between vents and return. This season and last. Never tested previously.

Last season large copper pipe at condenser was 64°, this season it's 76°. Both tested several times on different days. Never tested previously.

yes unit was on cool and fans were spinning when tested. Always waited at least 5 minutes after unit switched on to read temperatures.

This unit was installed in 2001, but is rarely used. It's never been charged since installation. Yes, I live in west central florida, but I use my heater more then the cool. It still cools me fine, but it does take slightly longer to cool the house.

Im affected more by humidity then heat. When Im uncomfortable, I set it to 2 degrees cooler then the ambient at that specific time. I might turn it off before the cycle completes or after it completes. Sometimes I let it cycle on and off a few times. Unless it's very hot and sunny outside, one cycle is enough to make me feel fine for a few hours.

When this unit fails, if i can't fix it myself, I might have to resort to window units.

I been reading a lot of service maintenance info online. Some of the things Ive checked:
no air leaks in duck-work.
vents and return are not blocked.
no fluid leaks.
I washed the condenser coils with foaming coil cleaner.
mtr spins free.
inspected electrical connections.
Duel cap read 38/5ufd and should be 45/5, so i replaced it.
I cant clean the evaporator coils with foam, but the top side looks fine.

Id like to do what i can to make this unit last.

tks

[100 td]

Generic info as you don't say what type of A/C, make or model.
What is it not doing exactly?
A/C's/fridges etc. don't EVER use refrigerant, that's why you can find 50 year old fridges purring along just like the day they were made. They only need refrigerant if there has been a leak, the leak needs to be found and repaired, not just some gas added.
After at least 15 minutes running with thermostat set low, check your temperature with a digital probe thermometer mounted inside the first air register in the circuit closest to the A/C unit and in the duct at the return air grill. Remove return air grill and remove filter and wash it.

[paulmars]

I didnt think that was important.
Both temps I listed indicate a problem. Especially the 2nd.
After 20 years, it would be very likely that its slowly lost some refrigerant. Especially considering everything else ive listed. Do I know that for sure? No, i dont, which is why I asked.
Wash a disposable filter?

[100 td]

You're obviously all over it, I'll leave you with it.