Air Conditioning - Pressure on AC drain. What causes this?

Hi,
I live in a 2 story town house.
I have ac unit on the second floor and drain goes from here to front of the house.
I recently had drain clogging problem and I use the vacuum to suck all the debris out of the line. Oh my god what a mess.
Anyways, I still have problem with water not going all the way down the drain.
When I apply high water pressure from upstairs to drain, water comes out from the drain fine. After couple minute of running water, water looks clean.
But when I apply light pressure, water never makes out of drain and flows back upstream.
Even when I try to blow air through the pipe air gushes back up.
What would cause drain to run backwards?? :confused:
Is there still clogging?

Please help me out.

Thank you

Have you checked the trap?

What would cause drain to run backwards


An improperly pitched line

Is it the primary or secondary line?

dun11 thanks for reply,

here are some pics, hopefully this helps lil bit to diagnose

main unit
http://img.photobucket.com/albums/v458/97bb6sh/IMG_0398.jpg

line where i cut and to clean,
http://img.photobucket.com/albums/v458/97bb6sh/IMG_0400.jpg

outside drain
http://img.photobucket.com/albums/v458/97bb6sh/IMG_0397.jpg

when u say trap, you mean by u shaped pipe?
i dont have any other access to the line because pipe runs from upstairs to front of the house outside.

anything i can try in this situation?

thanks

Have you checked the trap?




An improperly pitched line

Is it the primary or secondary line?

Yes sounds like a double trap but I dont c it , is it filling up pan under unit ? could it be primary drain pan leaking or freezing up and leaking due to to much water trying to drain ??

Sorry, been busy. Who ran that drain back up in your garden like that? That can't be helping. I would dig down outside and cut and clear the drain, I would guess the problem is there