The International Cessna^® 170 Association

Thanks guys, i'll be getting a few of the plastic ones from Spruce on my next order.

When that top tab is removed should i put the machine screw back into the top the of the master cylinder?

Yeah. the machine screw makes for another hole that needs plugged.
Instead of trying to drill a tiny hole in the plug, why not cut a groove down the threads just deeper than the threads? Easier to do with a razor saw (vs drill press and 1/32" bit) and serves the same purpose.

Thanks, i thought so.

the other plug in my right master cylinder is actually drilled on the side at a 45deg just above the threads. the left plastic plug is yellow and slotted like you describe and i can't quite make out where it is vented. assume it is probably like what you mentioned.