I have a clogged 3/4" pvc a/c drain that is plugged. The a/c is in the attic of a 2 story house and the drain pipe goes across the attic and down through the house to a ground level exit.

 

Some suggestions: Check the end of the line for debris first. If the clog is in a part of the line that is not easily accessible, try disconnecting the drain line at the a/c unit and flushing it with compressed air (you can obtain a portable unit for inflating tires at your local hardware store for around $30 and even if it doesn't fix the problem it is a nice thing to have handy in your car). If air goes through the line slowly or not at all, I would suspect hard water deposits (rust, calcium, etc.) have formed inside the line. In this case you could try flushing with vinegar or other mild acid solution until you get some flow. If air goes through cleanly, but not at the exit, you have a break somewhere and are likely doing damage to your walls. In that case you will be better off replacing the line. If there is a foreign object clogging the line, you might try flushing with a caustic agent such as lye. Just be careful that when you try to flush the line with anything that might build up pressure (air, acid, lye, or even water) that you stand clear of the opening you are accessing.