Last time this happened to me, it was due to defective dampers.
If the pump's internal rubber nodes were broken, the ink would just siphon through, and if you had a clog in the line, chances are the suction wouldn't pull enough ink from the heads anyhow. Check for clogs by moving the head aside and make a pull with a syringe from the line connected to the pump responsible for those cap-tops. If the ink drains, then a clog isn't your issue.
If that's the case, it's more than likely that your dampers aren't holding a vacuum, and just letting ink fall out, instead of holding it until a charge shoots it out.