sigh...

no, we are discussing MB fuel pumps...

Fuel Pumps come in the following varieties.

Centrifugal / Gear / displacement.

only displacement pumps will LIFT fuel, eg self prime with negative head.

This is why this type of pump is called a fuel lift pump, not a fuel pump....

MB Fuel lift pumps are displacement.

A displacement pump ALWAYS leaks, just like a piston and rings in a barrel always leaks compression... you leak this into the "case drain" back into the supply line, but, the actuator can still leak...

When a pump is powered by a cam on an actuator, it is always a displacement pump.

A diaphragm is what you use to prevent pump leakage getting into the sump.