Autobahnschreck

Above see the cooling circuit of the 991.1 (Motor type MA1.0x ). The expansion tank (6) has two vent lines, for both the front radiators, the cabin heatexchanger and thetwo gearbox heat exchangers. If the comfort valve (5) is manually opened, it allows for venting residual air into the expansion tank.
There are 3 disc valves, controlling flow to the heat exchangers for cabin heat and for gear oil /clutch fluid. A fourth disc valves controls the "short circuit" of the cooling circuit from clyinder outlet to the thermostate. This valve allows for rapid switching to complete flow across the radiators, if required. All four valves are operated by vacuum. The respective vacuum lines are controlled by the infamous electromagnetic "change-over valves". The COV get their vacuum from the pump driven by the right exhaust cam shaft, as does the brake booster. Further ( up to 6) COVs are controlling flaps for air filter, intake manifold, and exhaust flaps.
What happens if there is a serious vacuum leak ?
a) if between pump and any COV: 1) the brake booster does not work 2) exhaust flaps remain open after starting the engine. Probably warning: reduced/missing braking support .
b) between any COV and its user: the respective user/ valve remains in its position, even if required to operate. Exhaust flaps remain open (you should hear it, if you try to quiet them).
When the motor/gear oil/ clutch oil warms up and requires action of the respective COV without appropriate response, the DME will through a yellow error messsage ( " ..fault in cooling system,..service required".
A joke shop technicians sometimes play on their customers is not re-connecting a vacuum line to the exhaust flap actors. This results in the above quoted message, but only after about 15 mins. of driving after taking over your car.
Finally, what does this mean if your expansion tank leaks and you get the message "vacuum leak" ? I do not see an immediate connection between both problems. Even if the leaking expansion tank leads to boiling at normal operation temperature and thus insufficient cooling of the gear-/ clutch oil, the error message would not refer to "vacuum leak", but "cooling system"
From MD's I learned, people may have fleas and lice at the same time, i.e. , there may be different root causes for even unlikely symptoms you (or your car) suffers from.

dannyk304

Thank you once again for this very technical feedback and insight. In reading through several sources including what you sent, I'm also getting the impression that there may have been a root cause of failure in the vacuum system that led to a build-up of pressure that ultimately caused the expansion tank to rupture and leak. The culprit I keep coming back to is the engine coolant shutoff valve along the outlet to the radiators (#10 in the diagram), controlled by a COV. I read that if this stops functioning pressure will increase. I switched that COV with another when I did my initial tear down, so it's possible this valve itself is sticking or not being fed sufficient vacuum. Once I change the tank this weekend, and reset the code I'll take for a drive and if after 15 minutes the code pops up again, it'll be clearer to me that the tank was a casualty, not the root cause and I'll need to consider changing the shutoff valve. What's perplexing is that the car maintained nominal temperatures (PDK and Engine) while leaking profusely indicating that the cooling system was operating well excluding the tank leak. The car does have 105k miles and I drive her hard, it is possible the tank failure (a known issue in 2015 and 2016 (mine is 2016 with an end of 2015 build date) is my main culprit - I am being hopeful about that.