I have a 2014 Cruze 1.4T Eco MT and I installed the external check valve as well, routed to a throttle body spacer. That was almost a year ago with 116k miles on it, the dealer had replaced the valve cover once before and needed replacing again, which is what got me looking into why myself. With help from the Cruzetalk forum I found my check valve was missing. I replaced the tubing (GM tubing, not home made), replaced the valve cover, installed a BNR throttle body spacer to provide vacuum for the external check valve plumbing and now at 142k miles, no issues since. Time will tell but it seems like it should be a pretty permanent fix.
Overall I really like my car, but issues like this are really annoying. At least someone came up with a good aftermarket fix. Too bad GM couldn't make it right. I have had my water pump replaced (under warranty) and I've replaced a couple of coolant hoses and pipes myself. Each solved my coolant loss issues. Too bad so many of these cars have issues as they are a nice car otherwise.