The problem appears to be solved. I removed the delivery valve, hoping to find an obvious cause to my problem, but nothing jumped out. The only possible issue was a bit of "gum" in the nipple bore where the valve plunger rides. In addition to cleaning the valve, I stretched the spring slightly to give it a bit more enthusiasm when it comes to holding the valve closed. When reassembled, the hand pump developed substantially more pressure, easily activating the IP relief valve with every stroke, which had not been possible before.

Diesel911, the link to the thread describing your was very helpful. Prior to seeing it, I was set to order a new lift pump. Thanks, you probably saved me $140.