No I didn't check the rubber gromet yet,....

......and it's been a long time ago since I looked into a booster. At this time I even can't remember how it looks like and how it is accesible. I noticed some corrosion at the master cylinder base and along the booster where it mates the master cylinder.
I still have no time to get on it. Computers eat my life away. I hope they'll help me save time and $$ one day, hopefully soon.
I'll have to check the shop manual to get an idea where the grommet sits. Thanx for the pointer.