Even the old pre-OPD cylinders were SUPPOSED to be filled only to 80%. that is what the 80% valve (also known as the "spitter valve") was (and is)for.
When the cylinder is being filled, the 80% valve is supposed to be opened, and when liquid propane spits out the filling should be stopped and all valves closed.
On the new OPD valve equipped (or the old cylinders fitted with the new valves), the filling is stopped when the OPD shuts off or when the 80% valve shows liquid propane, whichever occurs first. Ideally, both should occur at the same time, but life is seldom ideal.
