TL;DR
The radius of curved rails should be reduce a tiny bit so that fluid wagons align with pumps.What ?
When a long train stops so that the tail curves around 180° the remaining fluid wagons do not align with pumps and the pumps do not engage.The same setup works fine with cargo wagons because their positioning doesn't have to be exact. I tried adding some straight or diagonal pieces and even an S curve into the turn to get the alignment back but didn't find any combination that would work. Looks like the radius of the curved rails should be reduces just a tiny bit so the wagons align.