What kind of fuel are you using?

If its e85 or alky the water temp needs to hit 200 before being parked for the day to burn the moisture out.

12an line to the breather helps. Its amazing how much water collects in those tanks.

I try to never start unless going to 200 to avoid milking the oil.

Adding a quart of Lucas seems to help with longevity as well.

Try one of those radiator caps that you can bleed the pressure off when warmed up.