[macssc911]

You can make a soap bubble solution water and dawn, you will know quickly how bad that leak really is. I have a background in HVAC and REFRIG, the typical leak source on a system that old is your evaporator , which is to coil in the air handler unit.. This is accessible behind panel door on that unit HVAC guy should have no problem finding a leak on that coil They have electronic leak detector that can find leaks less than an ounce a year. If you want to check it yourself use the soap bubble solution in a spray bottle and saturate coil make sure you cut power of to unit since your blower motor is close to evaporator. Give it some time for bubbles to form Have a good flashlight.
If by chance that is where leak is located. You can replace evaporator however don’t recommend spending that money better to replace with new system. Hope it’s the service valve as far as the cap HVAC contractor or HVAC supply house in your area should be able to help