I ordered a used OEM booster on ebay from a JY with good FB. I always forget to avoid rust belt places being that it's hardly an issue for me at home! This yard was in IL.
Anyway, the part was rustier than I'm used to and I noticed this rust in the "bore" of the booster. Hopefully you can make it out. This is the low point when mounted so it's where water would pool and the rest is not as bad.
Not sure if it's a concern?
I sprayed a little WD in there (reasoning WD tends to get along with most rubbers) and rolled the booster around a bit, but I don't actually know if anything "scrapes" those interior walls as the pedal is depressed? I need to watch some vids on exactly how boosters operate as I don't know precisely.
Anyway, the part was rustier than I'm used to and I noticed this rust in the "bore" of the booster. Hopefully you can make it out. This is the low point when mounted so it's where water would pool and the rest is not as bad.
Not sure if it's a concern?
I sprayed a little WD in there (reasoning WD tends to get along with most rubbers) and rolled the booster around a bit, but I don't actually know if anything "scrapes" those interior walls as the pedal is depressed? I need to watch some vids on exactly how boosters operate as I don't know precisely.