http://chevy.oldcarmanualproject.com/1929_57chevyparts/57cmpc0400.htm

If the gear inside is missing then it would definitely leak.
After removing the retainer, the gear holder can be removed (pulled out) but my memory is that the oil ring makes it a tight fit.
I don't recall an internal seal from gear shaft to holder ID but the shaft has a spiral groove.

The parts shown in the picture are examples only. Look up the parts you need.
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"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."