If you start with an empty master cylinder, full of air in other words, it
can be impossible to build any hydraulic pressure, especially if the rest of
the system is full. If you can't get pressure then you can't get fluid moving
through the system and you can bleed and pump for hours and get nothing.
Bench bleeding speeds the process, for DIY owners it sometimes happens that
they work like crazy trying to get the system to bleed, get frustrated, come
back the next day and the system bleeds fine. The MC will under some
situations purge itself of air and allow proper fluid movement.
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