In practice hydraulic systems leak. Even ones not designed to stop a truck.
But there are systems in production, usually only cost effective in large vehicles.
claims up to %25 improvement in garbage trucks.
http://www.boschrexroth-us.com/count...80_2012-10.pdf
%25 isn't small potatoes on a truck, but an electric hybrid has no problem improving mpg by %25 or more in a car sized vehicle (all this assuming a "dumb" driver).