Today in 1878, US Patent 206825 A was issued, an invention of John A. Prindle, for his “Improvement in Beer-Pump Valves.” There’s no Abstract, although in the description it includes this summary:

My invention relates to an apparatus for pumping fluids from casks, and is intended as an improvement on the device shown and described in Letters Patent No. 191,656, dated February 6, 1877, granted to Wm. F. Glass.

My improvement consists, principally, in the peculiar construction of the bung, provided with valve or valves, which close the bunghole hermetically and automatically by the pressure of gas from within, while air or gases may be easily forced through it into the cask, barrel, or other vessel.

It also consists in constructing the air valve proper of rubber or equivalent elastic material, forming it conical or conoidal, and causing it to rest upon a sharp-edged seat, whereby slight pressure will be sufficient to insure good results, and heavy pressure the same results, without liability of damaging the valve.