Quote:

Thanks, I was afraid I was tripping over something obvious!

My subject pump has the piston just barely sealing the passage at rest, so enlarging the existing hole can only be done to the rear in any event. A diametric increase is not a good idea since it captures the piston from cocking upward under pressure.
Some pistons may be possible to just reduce the length to allow it to reach full unmask position (or modify the retainer). Check the spring to make sure it doesn't bottom out 1st, etc.




I am assuming this pump you are working with is a
in pan pump so you can dump the oil