Air Conditioning - Air handler pan drain problem

Air handler problem: water did not drain from the pan in upright air handler. It is negative air pressure. Despite proper pipe drainage- u pipe plus two upright open pipe before and after u pipe. The install unit is “First Co.” model 30HBXB-HW -2.5T sitting on one end of the return box 10”x28”x40” and the other end is upright trunk 13’ long 10”x24” open to wall on return grill 16”x20”. To empty the pan I have to turn of the unit, then the suction force died and water flow frilly to the pomp and outside. Please help solve the mystery.

I don't think I understand your drain problem. Is the primary drain problem ?or the secondary drain problem ?? IS this a new system, How did it work before ?? Well, if you can post a picture, that will be help..

This is primary drain pan problem. Pan is factor install inside the unit. Unit is 3 years old
I have same unit on the attic in horizontal position and it working fine. Just that one in vertical position, when its on, not allowed water to leave the pan. Suction of the fen is so strong that preventing water to go out of the pen. When I turn of the fen water flow out.

I understand now. but I have never heard this type of problem that the suction force is so strong. usually, horizontal unit, the air blow the way which help the water goes out. I don't have a vertical unit, not sure how it is designed, hope some one else can help you..