Phu,
I'm not certain that your assumption of a hole in the flapper valve in cells 2,3, 6 and 8 is correct as I can't really see a need for it.
The man who can certainly answer your question is the great V1 oops. There isn't much about many of the smallest details of a Twin Otter that he doesn't know, and he wrote the FSI manual, and was been charged by DHC with keeping the manual updated when the aeroplane went out of production. Indeed he taught me everything that I used to know about the DHC-6 so I will be asking for a refund as here is question that he didn't give me the information about!
You should be able to get him via personal message. If you do please post the answer as you have me very intrigued now! Have a great time flying the aeroplane: you will never forget it!