On the Volkswagen Split Bus, the master cylinder with brake fluid reservoir is located under the cab floor. To fill or replace it, you don't need to be under the Bus - you can do it from the cab. Under the rubber mat is an inspection cover which can be removed by half a turn, giving you access to the brake fluid reservoir.
If the floor needs restoring, this cover probably won't be in very good condition either, and in many cases it isn't even there.
To make the cover more visible, it's painted black from the factory, but it's up to you whether you paint it with the body color or as it should be, black.
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