I bought a comfort maker 4 ton unit two years ago with a new 100 BTU blower unit. The outside unit was already charged with freon so we didn't add any after installation. I always keep the filter changed , but today its not cooling very good. The compressor is running the fan is running and the big low pressure line is sweating just a little. Maybe its low on freon, what do you think.

Sounds like a relay switch. Don't add more freon.

I really think you need to have a professional leak check the system. If there is a leak fix it and charge the unit properly.

The freon as you call it is in the unit when shipped but there could be too much or to little from the factory. Just putting it together and opening the valves do not work and is not accurate. Next I hope you pulled a good vacuum when you installed the unit. If you did not your probably have non condensibles in the system and that can cause huge problems and could already have.
The federal law states that you must have a license to work on units containing refrigerant 22 and also you need to have a recovery machine if the unit has to have a leak repaired.

I would call a professional and get there opinion. This could save the system from further damage.

PS the unit is running so there is no relay switch problem.

Sir, HVAC is not a guessing game.

As I mentioned yesterday you need a professional to look over your unit on site.

We have no idea what you are really having a problem with. That is because you cannot really give us the correct information we need to help you.

Most system that are installed by true non professional have major problems and that is what you are experiencing now. Everybody thinks they will save big bucks by doing it themselves and that is just not true if the system has problems like you are having now. Below is my recommendations.

Pull out the sheet you did your manual D on before you bought your unit. Recheck your calculations to see if you made a mistake in duct sizing. If you made a mistake in sizing your unit/duct system correct it. If you do not know what I am talking about call a professional.

This business is not a guessing game and I am not going to sit here a guess at your problems when I know the system is messed up to start with.
Bite the bullet and get it fixed correctly before you cause more damage than you may already have. It will be much cheaper in the long run that way.

This is the best I can do under these circumstances.