Auto AC ~ if I have a leak will there still be pressure in the line?

sorry but if you have a leak the freon will escape and eventually you will have no pressure.Now it has been a known fact that sometimes they can develop a very slow leak and cannot be detected and can be charged once a year,but I don't know what kind of problem you are having in order to help you.Other than what I mentioned in the beginning here.,,,,

The proper service of an A/C system requires some very specific steps to be taken.

First, you've got to recover any remaining refrigerant in the system.

When that's completed, finding and repairing the leak is the next step, then evacuate the system to remove any moisture.

Now is the time to add the proper amount of refrigerant and oil to the system.

Without a doubt, the best method is to take your vehicle to an independent shop and have a certified A/C technician service your system.

A/C systems are really not user friendly when it comes to DIY repairs since they require specialized tools, equipment and training to do the job properly the first time, without causing damage.

Don't waste your money on those recharge kits you can buy in auto parts stores as they don't deal with the primary problem.

The primary problem is that A/C is a closed system.

If your system needs recharged, it’s because there's a leak. The leak must be fixed before recharging the system or it's pointless.

Even if the problem with your system is one of a mechanical nature, the refrigerant recovery, evacuation, testing, sealing, recharging, etc, must be performed properly or damage may result.