I bled my brake today and a small amount of oil came out of the hole when I compressed the lever. The hole is just above the i on the word oil, the lever doesn't a have EBT port. The other lever has the same hole. I pushed the pistons back and took off the cap and put more oil in. When I tried again the lever went straight down to the bars.
Air escape port to allow bladder to deform into master cylinder reservoir.
When overfull you can force fluid past the rubber seal ion the top cap into this area. IIRC
only time I've had oil come out that hole is when I've pushed the pistons too far into the cylinders and overpressurised the system.
I assumed it was there to allow a bit of pressure equalisation above the rubber diaphram when it moves as the lever's pressed - Marta's had the same thing.
Ok thanks, I'll try again.