No water coming out of drian on rheem upflow 90% furnace.

It shuts the unit down cause it won't drain. I cut the drain line on the outside of the furnace below the condensate trap, and no water came out, and the drain is clear to the sump pit.

Water is backing up into the furnace, Only when I remove the bottom vacum line and suck the water out ( about 3 cups) every few hours, Then the furnace will run.

Any ideas? Thanks

Remove the drain hoses in the furnace and blow through them. Then blow through all connections where the hoses hook to. When blowing through the canister in the bottom of the furnace, plug one side while you blow through the other, and visa versa. Your plugging issues is in one of these area. If you clean them all, you will have cured the problem. Good luck and please let me know how things work out.

Why plug one side while blowing out the canister? Just curious

Why should I plug one side while blowing out the canister? What does that do?

If you don't plug one side with your finger, the gunk and water will just blow out the other side. Plugging one side while blowing on the other causes the stuff to go out to the drain pipe.