I did just did an experiment on a metro, when I pulled the hose from the booster there was a noticable noise of escaping air, so it seems likely that the check valve was external. I plugged the hose end removed from the booster (jammed in a round thingie of the right size) and took a test drive (no traffic, slow speeds).
Initially it felt pretty much like it does when the boost runs out. But I got a lot more confident with it as I used it. No problem locking up the tires, it just takes more pressure which legs have plenty of, especially in a panic
I am starting to think a big problem with running out of boost is the sharp transition to no boost braking. If you never have boost it seems pretty manageable. More leverage wouldn't hurt though, and of course your mileage may vary.