[Lang]

Here is are some diagrams of all the bits of a master cylinder.

This is the operation of the bypass port (the little hole you refer to)

When brake pedal is pressed, push rod moves the piston against the force of the spring to the right,till it covers the bypass port. Further movement of push rod creates pressure in the compression chamber. When sufficient pressure builds up,the fluid is forced into the lines via a fluid check valve which is not marked here though.

When the pedal is released,the spring pressure moves the piston back,but there is a delay in the return of the fluid from the lines back to the compression chamber. This produces a vacuum in the compression chamber and unless this is destroyed immediately,there are chances of air leaking into the system. Since air is compressible that is not at all desirable. To avoid this,we use the intake port. As soon as some vacuum is formed fluid from the reservoir enters the compression chamber. But by the time vacuum is destroyed, the fluid in the lines come back into the reservoir.

This extra fluid has to be accommodated because the compression chamber is full. This is avoided by the bypass port. The extra fluid coming from the lines passes to the reservoir through the bypass port.

Lang