A long time ago (high school actually) I installed a new set of valve covers on my car. The valve covers did not have breathers and I didn't think anything of it since that was how they came. The valley pan looked like a microwave popcorn bag after driving the car.

Engines build a fair amount of pressure in the crankcase, even if the rings are in good shape. It is fairly common to see engines push the dipstick out or push breathers off at WOT.