As Dave has mentioned, the early stovebolt hydraulic lifters had some design flaws that were corrected by aftermarket manufacturers later, including putting the bleed hole in the pushrod seat. The vent groove in the ball end of the pushrod also helps the lifters purge any trapped air. About the only worse designed hydraulic lifters I've encountered were in the Lincoln Zephyr V-12 engines from the early 1940's! Early Oldsmobile and Cadillac hydraulics in the 1950's had problems with lifters getting noisy, but it was finally traced back to a particular brand of oil foaming and causing the lifters to get spongy after a few miles at highway speed.
Jerry
