Adam,
My front-mounted Darley had the same problem. Turned out the clutch had completely disintegrated. Mine is off to the pump doctor now being rebuilt.
In addition to Bill's great advice, you might try calling Barton. I called the customer service number on the Darley web site and found myself talking to one of the Darley's -- he copied the manual for my pump, plus found all the paperwork on my pump dating back to 1952 when they sold it. He sent it all to me for postage!
You never know what a phone call or two might do for you -- give it a whirl.
John
