Shiny,
You have nailed it: the fluid cannot pass the check valve under gravity. The design of CVs varies as per brand, but the easiest to visualise & understand is the GM dual m/c where drum brakes are used. The CV is a simple rubber tip with a small spring that pushes the rubber into the brass outlet fitting on the m/c. No fluid exits until the spring pressure is over come, by some force on the push rod.
Pontiac Jim.
Later cars with disk brakes do not use a check valve [ & even some drum braked cars ], so these can be gravity bled.
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