It appears to have a defective washer. Not an ordinary O-ring unfortunately but rather a gasket with a "frill" on it. Their customer service number is 800-782-2385. I think it is a 104-557HC. I called on 7 January and they are sending and entire "stem" rather than just the O-ring. On 12 January I received a new complete "stem" for my faucet. Great customer service. I would have been happy with just the gasket but this will get it working again. B&K Mueller is a quality company. I'll buy and recommend their stuff for sure.
On further examination I found that the tubular shaft was cracked allowing water to enter it and spray out the knob. I soldered the cracks and it seems to work now.