Another option successful on my 2014 m5. Clamp the hoses
when the flaps are open. They will not go back to closed
IV hose line clamp or potato chip back clamp.
***UPDATE***
The Plastic potato chip bag clamps I used leaked by over night so now the opposite happens... They don't open until clamps are off. Would be better putting a short piece of clogged hose in between. Of course added as flaps are vacumed wide open.
Chris