(57) A shaft lip sealed is provided as an inlet shut-off valve seal in the working fluid
inlet line of a rotary piston pump between the first pumping chamber of the vacuum
pump and the inlet to the operating chamber. A piston valve is provided, having an
axially inclined radial face, for engaging this seal and cutting off flow therebetween
even under low pressures. This piston valve is actuated into sealing engagement with
the shaft lip seal when operation of the vacuum pump ceases and a pressure relief
vent is connected to the first pumping chamber.
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