I'm having trouble figuring out why there is a vaccum effect on a condensate line on a unit at a restaurant we work for. With everything closed up you can feel a slight suck at the end of the pipe where it drains out. The only way it stops is if you leave the access cover off the unit. This has been causing the unit to not drain properly and back up and flood the area. There is a trap in the drain line. Not really sure what else it could be.

Thanksfor the help!