Automobile Engineering in ME
The power source for a brake booster is

Exhaust manifold pressure
The pressure difference between the atmospheric pressure and the vacuum pressure in the intake manifold
Electricity
Hydraulic pump

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Automobile Engineering in ME
The function of a second ring is that it is a

Compression ring and aids the top ring in sealing and cooling
Back up oil control ring which will work in place of the bottom ring when it is broken
Cushion ring which prevents piston slap and knock
Back up compression ring which will work in place of the top ring when it is broken

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