Automobile Engineering in ME
The main characteristics of a maintenance free (MF) battery is that

A maintenance free battery requires little maintenance during normal use and it is sufficient to add water instead of an electrolyte containing sulphuric acid
A maintenance free battery has a relatively short shelf life when compared with standard batteries
Recharging of a maintenance free battery is neither required nor possible
Since it is sealed, the water in maintenance free battery is not lost through evaporation thus accordingly it is not necessary to top up the cells with water

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Automobile Engineering in ME
The function of a proportioning control valve (PCV) in a brake system is to

Reduce the brake fluid pressure when the brakes approach their lockup point
Cause less brake fluid pressure to act on the rear brakes than on the front brakes when the fluid pressure exceeds a predetermined level
Ensure that equal pressure is supplied to the front and rear brakes throughout every braking operation
Cause less brake fluid pressure to act on the front brakes than on the rear brakes when the fluid pressure exceeds a predetermined level

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