Unless someone has been monkeying with the lines they are NOT the problem. The problem will be either the center duct vacuum element, one of the defrost elements, or the recirc door element.
They all have vacuum on them in the AC position. A leak in any one reduces the available vacuum to the rest. The easiest test is to go behind the glovebox and with a vacuum diagram decide which lines go to these elements from the switchover valve block. Pull the lines one at a time and pull a vacuum (one can do it with one's own lungs easy enough, suck and put your tungue on it; if it leaks there will never be anything for the tongue to hold)
Bosch Master, ASE Master, L1
33 years MB technician