[Walt Fricke]

I'll believe the "fluid expands when hot and causes overflows" theory when someone does the calculations. Somewhere there must be a coefficient of thermal expansion for generic brake fluids. Which is not to say that keeping the fluid level down to half or so may not help control overflow no matter how caused. Would help with expansion, and should help with reverse flows from the MC - what gets pushed up when it shouldn't has to drain back down to refil the MC when brakes are released.

For me, the key fact is that with my fender side reservoir mount and non-power MC I never had any such issues. See how Sherwood's reservoir vents to the rear?

The failing MC seal effect is certainly real. In my case, the MC is but five or so years old, so that is not at the top of my list. However, if the rerouting of the overflow does not stop overflowing (and keeping the level lower as a next step) doesn't solve this, I guess it is MC rebuild time. Maybe the ATE MCs of today are not like those of the '70s?