The anti- suckback valve (called ASBV) is protected by a metal wire mesh filter. During a pump stop (for example due to shut down or a power failure), the ASBV closes the intake hydraulically & quickly. This prevents the pressure from rising in the connected chamber while the pump is vented at the same time. Any suckback of pump oil into the vacuum system is effectively prevented. This process operates under all operating conditions even when the gas ballast valve is open.