You are correct in that bumping the pump does not always prove the pump to be good, but it will 99% of the time and the extra work involved in setting the pre-pump up in a bucket of solvent does not warrant it. In all my years of wrenching on Volvos I have found it necessary to do it one time. The brand new AC pump I had just put in was no good. When set up in a bucket of fuel and run off of a battery it spewed as much or more fuel right out of the seams of the pump as it was pushing up the hose to the main pump. The method of bumping the pump and checking for flow worked good to diagnose the previous dead pre-pump. The brand new pump was airiating the fuel and you could hear/feel the air bubbles going through the feed line to the main pump. Problems like the one I had and the one you had are very few and far between but they do occur and when they do setting the unit up in a bucket of fuel or solvent is a very good way to diagnose it.

Mark