My '55 , as i am told, was a fire truck tanker also... but now has a flatbed. Mine has the break thing on the driveshaft no pump remains so not sure of that set-up. I have not run truck much---got it trailered here(drove it on and off) and it's been sitting 2 months so not real familiar with it other than plugs and wires and such. Mine has a 5spd w/4 spd brownie box. Look in my photo folder- there are a bunch of pics i took for guy with driveshaft questions- see if you get any answers from them. I would have to look more closely at my set-up to tell you anything. I'd love to see pics of the pump you speak of and controls.
https://www.youtube.com/watch?v=igIPSk0qqCQ .
https://www.youtube.com/watch?v=1ng3QHwOAik