You are manually opening the cartridge valves when you try to add suction from below the capping station right? You have to either open them in the service menus or remove the cover behind the cartridge slots and tape them open. If you don't you are creating a huge amount of negative pressure, and if you keep trying hard enough you will damage something, maybe a damper, or even sucking an o-ring out of place somewhere. Regardless you simply need to listen when you are trying to pull suction to hear where the air is getting in.
