If the battery helped the pumps then it sounds like low voltage. They could have been stuck, but sounds more like a voltage issue. One thing to check on these is the ground wire on the back of the alternator. For some reason, they galvanically corrode bad. I thought my alt was getting weak but when I went to check the ground wire, it broke off in my hand. Put a new end on it and never looked back. Hope your mechanic finds a simple solution.
I am still waiting for someone to explain how to reset the light. Maybe it will not reset until the fault is cleared?
1996 Prostar 205 LT-1