Gas and Oil Home Heating Furnaces - A/C Condensation Problem

I have two A/C units in the attic, both have a primary and secondary condensation drain. The primary drain for both units connect to each of two sink drains in the master bathroom - the drain pipe comes out of the wall and connects to the sink drain just above the sink trap on the sink side - a garbage disposal type fitting was used to connect the a/c drain. The problem with one unit is that it does not drain properly to the sink; i.e. this unit always drains out the secondary drain pipe. If I disconnect the primary drain connection at the sink and let it hang in a bucket, it drains fine. The sink drain is clear and the trap is clear. So I don't know why it won't drain when it is connected to the sink drain.

I'm thinking of reconnecting the primary drain on the other side of the sink trap - thinking the water in the sink trap is causing a "vapor lock" of some sort preventing the flow.

I'm looking options; I'm just guessing here.

thanks

sounds like when you drop the end of the primary drain (that sometimes does not drain) water then flows down it? could moving this termination down affect the slope up above? i would look at running those drains at a steep slope to the bathroom wall, especially if the drains have a horizontal run up above in the attic.

Thanks, the elevation is not changed - the primary drain pipe comes out of the wall as PVC and then it's just a rubber hose (about 12 in.) connection from the PVC to the connector above the sink trap. The PVC pipe is about six inches higher than the trap connection So all I'm doing is disconnecting the rubber hose end from the sink trap connection - for some reason it drains nicely into a bucket, but once it's connected back, the condensation starts coming out of the secondary. I've taken the trap apart and cleaned every inch of that connection path.