Yup, a wee round screen in the fitting. The fact that they get clogged once in a while proves that they are needed.
It's not terribly easy to get to and a real buggar to prove that it's causing your problem except by eliminating all other causes. Because there is gas in the hose, the bowls will fill up while you are digging out the driver to remove the drain screw to see if the bowls are empty, making this a lot harder to figure out than a clogged petcock strainer.
