60 Power Steeting

Chris, I don't think it is too much vent. I have pulled that bronze pellet completely out & not had the fluid come out. ~I have had a few in the past that I couldn't get clean [usually because someone had scraped on it] ~I did one about 6 months ago & decided to try something different & so far it seems to be working good. I punched that pellet out & then carefully broke the v shaped center out of an old bronze G.M. fuel filter, then used J.B. weld to glue the v shaped bronze filter piece into the center of the Deere power steering cap. I figured any oil that splashed onto the bronze would drip back into the reservoir & not blow out. So far I don't have any sign of fluid leaking out.
~With that hole you drilled any oil that splashes into the hole might get pushed out if there is a little positive pressure in the reservoir, or you are using the wrong type fluid & it is foaming inside.
~Try running with the pellet completely out & see if that helps, if so try the G.M. filter. If it doesn't work you can braze the hole up & re-drill a small hole.