Last week water found its way into my M5's brake booster.
The M5 forums document that people who have suffered from a similar problem will replace their brake booster to fix it. No mention is made of whether the brake booster is actually salvageable.
Is it possible that the brake booster can be salvaged by simply draining the water out?
What kind of damage could water do to the brake booster internals that would make it unfit for use?
Since that brake booster is very vital in the function of your brakes, I would HIGHLY suggest just replacing it; You could even get one from a junkyard depending on the year.
Once you get water into the brake booster, depending on the quality and coatings of the metal used; You can possibly rot the diaphragm or rust out the inside.
I just recently worked on a Hyundai Genesis and the brake booster failed due to faulty engineering. He literally ran into the back of another car because his leg strength wasn't even enough to slow the vehicle down.
I would replace it with a new one and tinker with the old one for fun if you're really interested.
