Reciprocating diaphragm or piston type vacuum

pump (Fig. 11.47(a and b)) These pumps operate very similarly to petrol and diesel engine fuel lift pumps.

When the camshaft rotates, the diaphragm or piston is displaced up and down, causing air to be drawn through the inlet valve on the downstroke and the same air to be pushed out on the upward stroke through the discharge valve.

Consequently, a depression is created within the enlarging diaphragm or piston chamber causing the brake servo chamber to become exhausted (drawn out) of air, thereby providing a pressure difference between the two sides of the brake servo which produces the servo power.

Lubrication is essential for plungers and pistons but the diaphragm is designed to operate dry.

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