Lots of motorcycle touring folks use the extremely high quality ORTLIB bags on the back of their motorcycles. Problem is, these bags do not offer an air purge valve so it's difficult to compress the bag down when it is loaded and you are rolling the top.
They do not offer a release valve from the factory. A quick trip to the Marine Supply store turned up a "Rule" through hull fitting (Model 60) and a Drain Plug from SeaFit (but any 1" drain plug would work fine).
Simply cut a small hole in the bag a bit smaller than the through hull fitting, insert fitting and tighten nut on other side. Remove drain plug, compress bag, reinsert watertight drain plug and go! I can now get the bag down about 40% smaller than I could before. PERFECT!