Put the verb in brackets into the correct form to complete

Put the verb in brackets into the correct form to complete passive sentence.

1.Discharge rate (to control) by throttling the discharge valve.

2.Clearances should always (to check) against the manufacturers manuals.

3.Oil cups must regularly (to fill) when the pump is in use.

4.Air reservoir (to install) on the discharge side to reduce pulsation.

5.Synchronizing gears usually (to mark) before removing the screws.

6.Multi-stage pumps (to require) to provide higher discharge pressures.